Fix obvious coding bugs in cord_cojoin/cord_join().

Remove unnecessary constants & arguments.
Keep an eye on the include dependencies becoming more messy.
Clear the O_NONBLOCK flag in the join thread, which
uses non-blocking API.
Suspend I/O in the main thread event loop for the
duration of replication join/subscribe.

tarantoolctl instance management script may run atexit()
handler a few times - and if the call stack leads back to lua_call(),
trying to free Lua state from within Lua call produced a crash.

Don't try to close lua_State from within lua_call,
this leads to a sudden death during an otherwise
orderly shutdown.

Use cached user credentials for global access checks and store
them in a separate fiber key.

This fixes gh-617 and speeds up privilege checks.

@todo: we need to add a linked list of all user sessions
to the user cache, as well as all user setuid functions.
These lists could be used to update privilege cache,
get/update per-user statistics, disconnect sessions
of a dropped user.

Encapsulate error handling in user cache search functions.

Rename user_cache_find to user_by_id() to be consistent
with func_by_id(), space_by_id() - API variant which
does not throw exceptions if an object is not found.

Make sure the tests pass under low memory and no memory overcommit.

Add diagrams/rewrite spec. Add snaps/xlog and replication commands overview.

Conflicts:
	doc/user/server-administration.xml

Conflicts:
	src/box/tuple.cc

Conflicts:
	debian/changelog
	debian/control
	debian/rules
	debian/tarantool-lts.lintian-overrides

Minor changes in cmake code style