OFStateManager: add function to lookup gentable entry by key

This change ports lookup functionality from the group table to gentables. This
is useful for when one gentable needs to refer to another, removing the need to
create a separate hashtable.
OFStateManager: add APIs to acquire/release gentable entries

These functions are analogous to the ones defined for the group table. As long
as a reference is held to a gentable entry it can't be deleted, so the private
data pointer remains valid. This allows the referencing code to store the
private data pointer in its own datastructures without risking a dangling
pointer error.
Merge into master from pull request #174:
Port lookup/refcount functionality from group table to gentables (https://github.com/floodlight/indigo/pull/174)