Don't use space with id 0.

Conflicts:
	src/box/sophia_engine.cc

Extract xlog.test.py into an own test suite, in preparation
for a split into many tests.
xlog.test.py restarts the server all the time anyway, there is
no point to keep all tests in the same file.

Deal with collateral damange of this seemingly innocuous move:

 * fix an assertion failure in user.drop(), user_has_data().
   The check was using a wrong index.
 * fix test-run.py to leave cores around in case of a server crash
   (in a friendly gesture to develoeprs, and joining the ranks of
   such extremely succefful applications as apport and systemd, it used to
   delete them)
 * update bootstrap.snap to provide format for _index space
 * make a bunch of tests safe to the order in which they are executed
 * use space name rather than space id for error messages more often,
   since space id is very volatile after introduction of max_id sequence
   for space ids.

Other changes:

 * Fix #708: catch exceptions in boxffi_iterator_next
 * Optimize index:get() to directly use index:findByKey() instead of
   box_process() + execute_select() + iterator->init() + iterator->next()

If we can't find the closest xlog starting before the current vclock
of recovery, return the first xlog with vclcok after the search pattern, to
continue recovery, even though missing some rows,
in case when of panic_if_wal_error is false.

A test case will follow in a separate patch.

Turn off jit for box.commit(), which yields.
A test case is missing since I failed to crash the
interpreter from the test suite (apparently it needs
a more complicated background logic than one can do in
a test case).

Fix a typo in bsdsocket.lua (a merge error).

Use engine flags rather than a virtual method to
check engine capabilities.
Use index name in the error message.

Conflicts:
	src/box/errcode.h
	test/box/alter.result

Do not involve in tarantool transaction log regarding
old_tuple (replace always return NULL).

This reduces transaction statement complexity from
tarantool side to a single sophia operation
(do replace instead of search + replace).

weak spots: txn_rollback_stmt()

2) added disabled error-injection test that crashes tarantool rtree index

Use a monotonically growing sequence for space identifiers,
to ensure they are never reused.

Update tests.

If socket is not a proper acceptable socket (-1, for example), break the
server accept loop, to avoid CPU hog.

Conflicts:
	src/lua/bsdsocket.lua
	src/sio.cc