Fixes #1659

No semantic changes.

This is a pre-requisite necessary to fix gh-1606
"visibility of changes in transaction in range queries".

With an index over all changes of a transaction it's now
possible to include these changes in a merge iterator.

* rename txv_tree_t to txvindex, use the name consistently

Never access the concurrent index from backend threads,
it now has no mutex to protect it. If tx index contains
an upsert tuple, read it in the tx thread, and pass
it ot the backend for merge.

* remove vinyl_cursor_next, rename vy_cursor_conext to vy_cursor_next

Eliminate a useless wrapper around vy_index_read().

Use vy rather than vinyl consistently.

* account all reads together for now
* we will need new statistics anyway when we have a new cursor
implementation

* VY_TASK_BRANCH -> VY_TASK_DUMP
* branch -> run

Fixes #1639

This methods are used for opening and closing
indexes instead of making it in Index constructor.

Fixes #1640

Out buffer, must be at least UUID_STR_LEN + 1 length.

The C code uses memcmp() to compare UUIDs for server selection
in replication bootstrap, but in lua there is no way to do it,
because UUIDs there are represented as strings.

This patch fixes the C implementation so that it will behave
as if UUIDs are compared as strings.

Fixes #1598

* Rename fields types in data dictionary and test suite:
 - 'num' => 'unsigned'
 - 'str' => 'string'
 - 'int' => 'integer'
* Force coversion to lowercase
* Add aliases to space:create_index():
 - 'str' => 'string'
 - 'num' => 'unsigned' -- logs "field type 'num' is deprecated" warning
 - 'uint' => 'unsigned'
 - 'int' => 'integer'
* Add prefix to `enum field_type` members
* Add upgrade_to_1_7_2() script

Fixes #942
Fixes #1534

* use vinyl_index instead

* provide savepoint API, for statement-level rollback
* don't implicitly rollback entire transaction in console (this
  breaks a fix for gh-817 + memtx, but it's the least of two
  evils when working with console - not usable transactions in vinyl vs
  inconsistent behavior of transactions in memtx

* prepare for removal of tx_index, move all dependent declarations
up for that purpose
* vinyl_rollback returns void

Fixes #1664

Fixes #1602

Fixes #1663

Change mkdir() mode to 0777 to be consistent with WAL writer and
snapshot dumper which use 0666 for files.
umask is applied as usual.

Fixes #1571

Try to increase readability of vy_apply_upsert().

Fixed incorrect update ops checking in vinyl
and memtx.
Fixes #1650

https://github.com/tarantool/build/issues/16

* the conflict manager index is used only in the tx thread.

Don't maintain the number of writes, it's broken in case
of a savepoint rollback, and is implicitly known from a non-empty
write log.

* yes, they shouldn't have been there in the first place