table: improve learner table timers

table: improve learner table timers

Previously, on lookup hit, the hit key had its timer automatically
rearmed with the same timeout in order to prevent its expiration. Now,
a broader set of actions is available on lookup hit, which has to be
managed explicitly: the key can have its timer rearmed with the same
or with a different timeout, or the key timer can be left unmodified.
The latter option allows the key to expire naturally when the timer
eventually runs out, unless the key is hit again and its timer rearmed
at that point. Needed by the TCP connection tracking state machine.

Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>