ssa.c: In ssa_downstream_handle_epoch_publish, check epoch against DB_EPOCH_INVALID

even though DB_EPOCH_INVALID is 0

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: In ssa_downstream_handle_epoch_publish, validate published epoch length

log a message if not what is expected and don't RDMA WRITE

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