openflow_input: remove "_t" suffix from enum names
add "stable" parameter to enums

This parameter gives a hint to the backend that the enum members have the same
value in all OpenFlow versions. It's enforced when generating the unified IR.

The stable parameter is added to the BSN port/vlan/vrf stats extensions.
java_gen/pom.xml: Bump version to 0.4.1

Due to renaming several enums.
Merge into master from pull request #334:
add "stable" parameter to enums (https://github.com/floodlight/loxigen/pull/334)