cbus: introduce lcpipe - light cpipe
Introduced a new type of cbus pipe - lcpipe. The current pipe in the cbus - cpipe, has a number of limitations, first of all - the cpipe cannot be used from the 3rd party threads, cpipe only works as a channel between two cords. That why lcpipe is needed. Its main responsibility - create channel between any thread and tarantool cord.
Internally lcpipe is a cpipe, but:
- on flush triggers removed, cause triggers use thread-local mem-pool, this is not possible on a third party thread
- producer event loop removed, cause there is no libev event loop in third party thread
Also, lcpipe interface is exported to the outside world.
NO_DOC=core feature