Make box.slab.stats() output CALL compatible.

Reflect in slab stats the quota size and index memory statistics.

arena_used_ratio - how much of arena is used
items_used_ratio - how much memory is actualy used for items,
given that there is some fragmentation. Doesn't reflect index pages
arena_used - how much of the quota is used (i.e. how much memory
is used for index and tuple data)
arena_size  - this is the same as box.cfg.slab_alloc_arena.
quota_size - how much of the quota is defined (box.cfg.slab_alloc_arena)
quota_used - how much of the quota is used (reflects the memory taken
by all slabs, including slabs in caches)

Incompatible change:
Extract detailed slab statistics to a separate call, box.slab.stats()

Implement 'strict' module, which prohibits use of undeclared global
variables.

require('strict').on() turns strict mode on
require('strict').off turns strict mode off

Turn strict on by default in debug build, off in release builds.

Don't subtract con->parse_size on every call to iproto_connection_close(),
there may be many calls if there are many outstanding requests when
the socket is closed.

Conflicts:
	src/box/iproto.cc

There is still a valid case for fork() after start, it's box.coredump().

The assert that replaced recovery_atfork() call would sporadically fail
even in our own test suite.

Revert the change.

Any function of Lua log module would first evaluate the call stack,
and only then invoke the logger function -- even if the text is
then sent to /dev/null because log level is too low.

Short cut the log call in case the log level is low and the message
is muted.

Check ev_async_pending() rather than state of the pipe to find
out whether we need a syscall. On a simple read-only test pipe->pipe
is empty while there is already a pending event more than a few times a
millisecond.

At the same time, checking ev_async_pending() is more expensive, since
it's an atomic variable, but overall performance increased
after the patch, so the cost seems to be justified.

Use staged mutex lock/unlock, cool off the WAL mutex

Since we no longer fork to make a snapshot, we don't need
an atfork() handler in box. We still need atfork() handler
in the app server to handle background=true option.

See also gh-517

refactoring: extract wal writer, rename replication.[hc] -> relay.[hc]
Extract the write ahead log into its own C file, rename
replication.[hc] to relay.[hc]
Cleanup headers.
No semantical code chagnes.
wal: begin changes necessary to switch to the common bus

Implement a generic inter-cord communication bus and use it
in separating network I/O and transaction processor.

Split all request processing work into network thread part
and transaction processor thread part, and make these
parts multi-threading safe.

Generalize fiber pool into an own data structure and API.

Misc cleanups.