The cache iterator restoration function is muddy and suboptimal:

 - If last_stmt equals NULL, it doesn't attempt to change the iterator
   position although it should restart the iterator. Moreover, it
   doesn't even check EQ condition in this case, i.e. it may return a
   statement that doesn't satisfy the search criteria.

 - It may restore iterator position to last_stmt, but the restoration
   semantics mandates that the returned statement must be strictly
   greater than last_stmt if the iterator was restored.

 - If the iterator was invalidated, it first positions the iterator to
   the last known position (curr_stmt) and then steps back to the
   statement closest to last_stmt although it could jump to last_stmt
   immediately.

Let's rewrite it.

vy_merge_iterator_squash_upserts() returns an UPSERT, which is then
converted to a REPLACE by vy_read_iterator_next(). Fold the latter into
the former to make the workflow easier to follow. Also, remove useless
suppress_error argument.

Not only restore() callback can restore the iterator position after the
source changed - next_key() and next_lsn() can do that too, which brings
additional complexity. Do we really need it? Not really:

 - We can avoid handling restoration in next_key() by simply calling
   restore() before it. This is cheap as restore() is a no-op if the
   source was not changed.

 - Regarding next_lsn() - when it comes to calling it, we iterate over
   sources from the most recent one to the oldest, which means that the
   only sources out there that may yield, on-disk runs, are guaranteed
   to be iterated last, after all mutable sources (txw, cache, mems), so
   none of the mutable sources needs to be restored in this case.

Simplify the read iterator as per above.

Currently, ->next_lsn() may be (and actually is) called right after
opening the iterator, which means that apart from advancing the iterator
to the next version of the same statement it has to handle the case of
starting iteration. This complicates the callback semantics so let's
require ->next_key() to be called first.

Closes #2774

We generally do not allow to drop objects that have dependents.

In contrast to PostgreSQL, this method doesn't make sense in Tarantool,
because we don't have sessions.

This patch implements a new object type, persistent sequences. Sequences
are created with function box.schema.sequence.create(name, options).
Options include min/max, start value, increment, cache size, just like
in Postgresql, although 'cache' is ignored for now. All sequences can be
accessed via box.sequence.<name>, similarly to spaces. To generate a
sequence value, use seq:next() method. To retrieve the last generated
value, use seq:get(). A sequence value can also be reset to the start
value or to any other value using seq:reset() and seq:set() methods.

Needed for #389

Currently, it is done by space_cache_replace/delete which violates
incapsulation. Let's introduce a trigger that is fired after a change in
a space definition is committed and use it to propagate changes to Lua.
Patch by @kostja.

Implement a helper function to swap space triggers.
Patch by @locker.

Speed up zstd as much as possible.

Update zstd submodule to v1.3.1 version.

Needed to implement SMTP client on the top of libcurl.

See #2704

We can't call initial() from 1.7.6+ because of the space format checks.
Port initial_1_7_5() from 1.8 and use it to boostrap.

Coverity Scan has found a possible bug in a CSV parser - dereferencing
of a null pointer.
But it is not possible. Lets explain:

According to Coverity, the null dereference is in CSV_IN_QUOTES
processing in csv_parse_impl.
But bufp (nullified pointer) can be set to null only in two cases:
CSV_NEWFIELD and CSV_OUT_OF_QUOTES.

In a case of CSV_NEWFIELD the parser sets the state to CSV_LEADING_SPACES and
on a next iteration it sets bufp to not null.

In a case of CSV_OUT_OF_QUOTES the parser can nullify bufp only
if is_end_of_line was true at the end of 'out_of_quotes' processing.
But it means, that is_end_of_line was true at the beginning of
processing too, because CSV_OUT_OF_QUOTES does not modify it.
And at the beggining, if is_end_of_line was true, then the state
is set to CSV_LEADING_SPACES. And on a next iteration bufp are set
to not null.

Closes #2692

Closes #2754

Needed for #2754

sc_space_new return value always is unused. Make sc_space_new
return void.

Needed for #2754

Version is needed in #2754 to ignore space format,
if schema version < 1.7.6.

Needed for #2754

Needed to get format fields during alter in #2754

No semantic changes.

Needed for #2754

Needed for #2754

Needed for #2754

Needed for #2754

Minor renames

Travis images are broken by design.

Closes #2069

Ubuntu Trusty on Travis is buggy.

Fixes #2752