Export memsize() in Lua (part of the original patch by @unera).

Conflicts:
	src/lua/bsdsocket.lua
	test/box/bsdsocket.result
	test/box/bsdsocket.test

Fix link to homebrew formula

Change local hot standby to run in a fiber, not out of
event loop hooks.

admin_credentials need to have correct credentials of the superusers,
but they are initialized before the user subsystem is initialized.
Initialize them manually.

ER_CLUSTER_ID_MISMATCH outputs two UUIDs, while uuid_to_str can
only work with 1.

Add replication status to box.info().
When replication is off, box.info.replication.status is 'off'.
Other allowed statuses are:
    - 'connecting' - trying to connect to the master
    - 'connected' - connected, but nothing read yet,
    - 'running' - successfully received first rows
    - 'failed' - network error
    - 'stopped' - apply conflict, replication needs to be restarted

Move recovery_last_update and recovery_delay to section 'replication'
in box.info(), replace the first with 'lag' variable, indicating
the time difference between the currnet time and row time of the last
row, and 'idle' variable, indicating the difference between the
current time and the time of last event from the master.

Also output the text of the last error when it happens and
stops replication.

Make vclock output in box.info more compact.

Remove 'sophia' section from box.info, it's big, and we need
to solve the problem of status systematically for all engines.

Fix crashes when trying to stop failed replication: fiber_cancel()
of a dead cancellable fiber would try to schedule it and crash.

Conflicts:
	src/box/memtx_hash.cc
	src/box/memtx_tree.cc
	test/wal/oom.result

This patch also extends API of box.session.su():

 - box.session.su(username) - changes user to `username'
 - box.session.su(username, fun, ...) executes `fun' with permission of
   `username'

This feature is needed to fix a possible security hole with passwd().

Keep compatibility with Tarantool 1.5

In JOIN we pass UUID in, when replica follows the master,
we pass integer server id in. Convert integer to string for output.