ssa.c: Add log message before and after closing rclose thread pool

After commit 54326af6e7d7352953507bfbe22608ed72890376,
g_thread_pool_free could take a long time because it waits
for finish all rsocket closings.

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