ssa.c: In ssa_access_send_db_update, clear epoch in DB update message

to indicate epoch is "not used" in this variant

Resolves cosmetic issue in log where epoch is used (ssa_upstream_send_db_update)
ssa_downstream_handler: SSA DB update from access: rsock 53 GID fe80::2:c903:ea:5b11 ssa_db 0x7fa8f8006450 epoch 0x7fa8f8006450

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