OFStateManager: add API to get table ID assigned to a gentable

The table ID is used on the wire when referring to one table from another, such
as with of_action_bsn_gentable. The code parsing this action needs to know what
table ID was assigned when a gentable was registered.
Merge into master from pull request #182:
OFStateManager: add API to get table ID assigned to a gentable (https://github.com/floodlight/indigo/pull/182)