box.ipc: add :close method. #158

+ use luaL_ref/luaL_unref to store lua scalar instead lua metatable.
+ add :is_closed method.
158-box.ipc.close: minor changes (code style)
MMerge branch '158-box.ipc.close' into stable

Conflicts:
	doc/user/stored-procedures.xml
Merge branch 'stable' of github.com:tarantool/tarantool into stable
Merge remote-tracking branch 'origin/stable'

Conflicts:
	doc/user/target.db
	src/ipc.cc
	test/box/ipc.test