Introduce host table, make tag use it

Introduce host table, make tag use it

Add a host table similar to the flow table. A hash using fine grained
locking. Flow manager for now takes care of book keeping / garbage
collecting.

Tag subsystem now uses this for host based tagging instead of the
global tag hash table. Because the latter used a global lock and the
new code uses very fine grained locking this patch should improve
scalability.