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.

Add a test case.
Rename fiber.test to fiber_stress.test
Change cxception propagation so that exception is fiber-local,
not thread local.
Added exception propagation to fiber_join().

Rename fiber_zombificate() to fiber_recycle().
Don't double-initialize same members of the fibers.
Introduce a function which contains common initialization
of fiber_new() and fiber_recycle().
Use a common naming scheme for lists of dead, alive and ready
fibers in a cord.

Reduce size of the title to fit name/ip:port into 32 characters

Remove extra log message.

Remove explicit fiber call stack from the cord.

Fix backtrace() to work correclty with an optimized (-O2) build.

Avoid overwriting tuple data with information necessary for
smfree(). Only use a single pointer to store delayed free context.
Add a test case.

Minor cleanups/comments/renames.

Fix box.schema.user.exists(nil) box.schema.role.exists(nil)
and box.schema.func.exists(nil) to return false (PeterG report).
Add tests.

Conflicts:
	test/box/cfg.result

Some settings (too_long_threshold) were not set at all
after the change that removed initialization of all options
from box_init().

Always set all dynamic options after load_cfg().

Refactor output of box.slab.info() to be more readable (as readable
as I could make it taking into account it's a Lua table).