ssa.c: Wrap pushing rclose to the thread pool into an function.

 The commit adds a new function ssa_close_rsocket. The function
 could be either rclose pushing to a separate thread or sequential
 call to rclose depending on value of RCLOSE_THREAD_POOL_WORKERS_NUM
 define.
 RCLOSE_THREAD_POOL_WORKERS_NUM > 0 - use thread pool for rclose
 RCLOSE_THREAD_POOL_WORKERS_NUM < = - call rclose in the same thread.

 RCLOSE_THREAD_POOL_WORKERS_NUM is configurable from compiler's
 comman line(see: a51dc18a7eb7fad932e0b9d1d4e40ff01d591a9e).