A fix for https://bugs.launchpad.net/tarantool/+bug/992171
"SAVE SNAPSHOT process (dumper) does not respond to signals"

Reset the signal mask/handler after a fork to their defaults.
Make sure that when the snapshot dumper process is terminated
with a signal, we report it back to the user.

No test case since it would be hard to do a concurrent
test in our framework yet.

When salloc_init() fails, we call tarantool_free(),
which calls fiber_free(), which tries to destroy the fiber registry
hash, which has not been allocated.

Check that the hash has been allocated, before trying to destroy it.

Fix bug https://bugs.launchpad.net/tarantool/+bug/990728
"box.space:truncate() is a CPU hog".

The old truncate algorithm would restart iteration
from the beginning to delete every tuple.
With HASH index it was especially slow, since
it would have to skip more and more empty slots
to find a non-empty slot for deletion.

Rewrite the truncate() to walk linearly
over all elements and delete them.

This works both with HASH and TREE indexes.

On the same token make truncate multipart-index
aware (was missed in multipart-pk patch).
Add a test for that.

To work with multipart keys efficiently,
implement tuple:slice(). A slice returns
a linear set of fields from a tuple.
Convenient to retreive a multipart key or
tuple tail. Document tuple:slice().

Convert box.lua to box.lua.c and compile it in.
Fix broken out-of-source build of luajit, by adding
a few more generated files to touch-nocreate rule.

Replacing salloc() with malloc() found a bug triggered
by multipart key patch:
we don't always check for key cardinality.

Make the test more explicit.
Remove old/new config checking, which is currently pointless.

When the slab alloc arena is fully used, and we try to
truncate a space, we could crash, since

a) we never check return value of salloc()
b) we used to salloc() an iterator for truncate(), and
it would return NULL, and we would try to access it without a check.

The fix is:

- to throw exceptions directly from salloc()
- to not use salloc() for iterators, since we still should
be able to truncate a namespace when the slab alloc arena is
full.

This version of libev optimizes async events significantly.

Fix bug
https://bugs.launchpad.net/tarantool/+bug/983094
"Valgrind tests were broken by WAL writer patch"

Keep track of WAL files opened for write
in recovery

Extend atfork handler to clear the wals,
to avoid collision with reuse of recovery
to save a snapshot.

Move some WAL-specific configuration settings
from wal_class to recovery, to avoid layers
of indirection when doing RELOAD CONFIGURATION

Conflicts:
	mod/box/box.lua
	test/box/lua.result
	test/box/lua.test

tarantool_lua_dofile function was rewritten.

Fix brokken static compilation of the client (we'll have
to find another solution for Darwin).

Make snap_io_rate_limit and io_collect_interval dynamic.
Add a test case.
Update the user guide.

Review changes:
  * tarantool_lua_load_startup was renamed to tarantool_lua_load_init_script;
  * TARANTOOL_LUA_STARTUP was renamed to TARANTOOL_LUA_INIT_SCRIPT;
  * init.lua int the test suite is optional now;
  * Additional comments was added.