ssa.c: Modify downstream handle query routines to use proper DB

SSA DB from ssa_conn struct if available otherwise PR DB if available

Also, when update DB message is received by downstream handler
from access thread, save ssa_db struct pointer passed in the
ssa_conn staruct so that the downstream query handling can use this
if available

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