Implement box.once().

Right now there is a race in box.once(), this is somethign we'll
need to fix when we have master election on simultaneous bootstrap.

The patch merely "establishes" the syntax which is going to work
most of the time.

When handling join, an error from box_on_cluster_join()
was ignored and not sent to the client, so the client
believed that the JOIN was handled successfully and proceeded
with SUBSCRIBE.

Before sending OK to join, execute box_on_cluster_join(),
if it fails, the client sees an error and stops.

Make ER_REPLICA_MAX a logged error.

No test case since the test requires a fail-stop test of the replica,
which is clumsy in our framework.

Don't log exceptions of dead joinable fibers - it will
be logged by the caller, so avoid double logging.

recover_remaining_wals() is now more like an iterator,
vclockset_match() is only called when there is no current WAL.

Add a test case for gaps in LSNs.

Remove the code which existed at times when we weren't
running everything inside an event loop, so required
alternative ways of signal handling in and out of an
event loop.

Reset signal handlers when handling fatal signals,
to avoid an infinite loop.

In panic_on_wal_error=true mode, we only skip ClientError errors,
but not SystemError errors. An error to allocate a tuple is
a recoverable error, so use ClientError for it. This way,
at recovery, tuples which can't be allocated are skipped.

io replaced with fio

Add a directory watcher, in addition to the file watcher.

Chagne wal_dir_rescan_delay default value to 2, since now it's
merely a backup.

When a new log file is created, replication relay may notice
and try to open it before it has a valid header. In this case
the relase is lead into believing that the log file is corrupt.

When opening a new xlog file, first create the file with .inrpogress
suffix, then write the header, thus get a good file, and only then
rename it to a name without a suffix, so that it's noticeable by
the relay. This prevents sporadic failures of replication with
message "failed to read log file header".

The problem was spotted in Avito after a recent upgrade.

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

Fix a crash in rollbackStatement() when rolling back a
statement trying to access a non-exissting space.

space_cache_find() is called before txn_begin_stmt(), thus
there is no statement object, nothing to roll back.

Remove iditioc tests from the suite.

Change the logic of recovery_finalize() to open
the next xlog if the last xlog had zero rows.
Add a test case.

Remove recovery->finalize, use recovery->writer instead.