Implement box.session, on_connect and on_disconnect triggers.

Since connect/disconnect events in iproto run outside
fiber context, queue execution of on_connect/on_disconnect
triggers through the request queue. Generalize the request
queue for that purpose.

Implement a framework for on_connect/on_disconnect triggers
and Lua bindings for it.

Add tests and docs.
Merge branch 'stable'

Conflicts:
	doc/user/stored-procedures.xml
	include/box/box.h
	include/mhash.h
	src/fiber.m
	src/tarantool.m
	test/box/fiber.result
	test/box/lua.result