java_gen: small improvements to gen_table/tlv extensions

 - move bsntlv classes to dedicated package/factory
 - change type of of_bsn_tlv_vlan_vid to VlanVid
 - change type of tableId to new typesafe constant GenTableId
Merge into master from pull request #173:
java_gen: small improvements to gen_table/tlv extensions (https://github.com/floodlight/loxigen/pull/173)