If no password is set for GUEST user, allow a client
to authenticate to GUEST.

For all other users, no password means that it's only possible to become
such user with session.su() or inside a setuid function.

The exception for GUEST user is useful for connection
pooling, where we would like to "reset" a connection to its
default, non-authenticated state, when returning it to the pool.

Weed out 'box' data type for now. A special name can backfire
in the future, when we exteend HASH/TREE/other types.

Add checks for uniqueness.

Disable check of key parts, it was not used any way,
since an explicit check is made in initIterator() and findByKey().

Mark sophia engine as  not supporting RTREE.

Fix a bug in iovec_join which would overwrite data of
the first iovec with subsequent iovecs.
Double the default buffer size.

Conflicts:
	src/box/tuple.cc
	src/box/tuple.h

Conflicts:
	src/say.cc

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