ssa.c: Skip PRDB calculation in case of SMDB update in process

Problems:
Access can receive SSA_CONN_DONE message from downstream while
an SMDB update is in process. Access could crash because it may
touch SMDB during updating since PRDB calculation requires a
stable SMDB.
Also, downstream doesn't accept incoming ACM connection in case
of waiting for SMDB update. It checks Access's state using flags
access_update_pending and access_update_waiting. Access may
change the state after the flags checking, but before the
SSA_CONN_DONE message processing.

Solution:
1. Access will check the flags before PRDB calculation. The
   flags can only be changed in Access thread. So, Access knows
   that there is no SMDB updating at the time.
2. If Access is waiting for SMDB update, it skips the PRDB
   calculation but adds a new consumer to the internal list.
   Only Access touches this data structure.
   PRDB for the new consumer will be computed when ongoing
   SMDB update is indicated by receipt of SSA_DB_UPDATE message.
3. Downstream accepts new PRDB connection even if SMDB update
   is in process.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Remove no longer needed log message in ssa_downstream_svc_server

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