table: support learner tables

table: support learner tables

A learner table is typically used for learning or connection tracking,
where it allows for the implementation of the "add on miss" scenario:
whenever the lookup key is not found in the table (lookup miss), the
data plane can decide to add this key to the table with a given action
with no control plane intervention. Likewise, the table keys expire
based on a configurable timeout and are automatically deleted from the
table with no control plane intervention.

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