ssa.c: Configure number of workers in rclose thread pool

Override RCLOSE_THREAD_POOL_WORKERS_NUM in compilation command line
using CFLAGS as follows:
./autogen.sh  && ./configure CFLAGS="-DRCLOSE_THREAD_POOL_WORKERS_NUM=3

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Print number of rlose thread pool workers to ssa log

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>