ssa.c: Resolve rsocket id race between access and downstream threads

SSA_DB_UPDATE message sent from access to downstream includes
rsocket id. The id is a key in fd_to_conn lookup table.
Using the id downstream obtains pointer to connection structure.

A client may disconnect from AL while SSA_DB_UPDATE message with its
socket number is waiting in the socket or in db_update queue.
Other client may connect to the AL and recieve the same rsocket id.
In this case, downstream may obtain wrong connection.

GID checking avoids using wrong connection. GID is unique for
a client and can't be changed.

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