MINOR: hlua_fcn: add Patref:commit() method

MINOR: hlua_fcn: add Patref:commit() method

commit() method may be used to commit pending updates on the local patref
object:

hlua_patref flags were added:
 HLUA_PATREF_FL_GEN means the patref object has been updated
 and it is associated to a new revision (curr_gen) in order to prepare
 and commit the pending updates.

upon commit, the pattern API is leveraged with curr_gen as revision to
commit new object items. Once commit is performed, previous (pending)
revisions that are older than the committed one are cleaned up (similar
to what's done with commit on the cli). Also, Patref function APIs now
take into account curr_gen to perform lookups.