Specific flow table list changes to track in use entries.

This optimizes the flowtable match operation by avoiding iterating
over unused flowtable buckets.
twise -> twice.
calloc to realloc, freed the fts_list memory in flowtable_destroy.
Merge into master from pull request #17:
Specific flow table list changes to track in use entries. (https://github.com/floodlight/ivs/pull/17)