ssa.c: Add upper limit for number of access workers

The limit could be configurable from compiler command line.
Example:
./configure CFLAGS="-DMAX_ACCESS_POOL_WORKERS_NUM=2"
By default, the number of access workers is unlimited.

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