ssa_db.[c h]: add global epoch for ssa_db

The following changes were done:
 - An epoch field was added to 'struct db_def'
 - ssa_db_get_epoch() was added (returns host order ssa_db epoch)
 - ssa_db_set_epoch() was added (no return value)
 - ssa_db_increment_epoch() was added (returns new epoch value
   in host order)
 - ssa_db_helper was properly updated

* please notice that previous SMDB/PRDB dumps will not
  be compatible with current ssa_db_helper version,
  since an additional field (epoch) was added to db_def.

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