Complete implementation of generic flowtable.

Generic flowtable hash flow mask as hash key and specific flowtable with
flow key as hash key. Related changes in OVSDriver, flowtable unit test
and benchmark testing.
Review comments addressed in full generic flow table implemenration.

Mergerd flowtable_specific with flowtable_generic_entry.
Renamed flowtable_generic_entry as flowtable_specific.

Also changed generic flowtable as flowtable at appropriate places.
Merge into master from pull request #15:
Complete implementation of generic flowtable. (https://github.com/floodlight/ivs/pull/15)