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