plugin: Add support for SMDB that does not change between updates

In case of no changes in SMDB, it is not being
destroyed and SSA_DB_UPDATE message is sent
with SSA_DB_UPDATE_NO_CHANGE flag.

The issue was found during integration on KODIAK
cluster, and caused the core layer to crash.

(gdb) bt
0x00007fd57f27ebd6 in ssa_extract_update_ready_process (p_osm=0x7fff990ee230, outstanding_count=0x7fd57dc43d80) at src/core.c:967
0x00007fd57f27f1bb in core_extract_handler (context=0x7fd57f4a7ea0) at src/core.c:1127
0x00007fd58227de9a in start_thread (arg=0x7fd57dc44700) at pthread_create.c:308
0x00007fd581faadbd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
0x0000000000000000 in ?? ()

Pointed out by Lenny Verkhovsky <lennyb@mellanox.com>
Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>