openflow_input: add bsn_gentable extension and tests

See the documentation in openflow_input/bsn_gentable.
loxi_ir: support of_checksum_128_t
pyloxi: support of_checksum_128_t
java_gen: support of_checksum_128_t
loci: support of_checksum_128_t
frontend: recognize key_length as an OFFieldLengthMember
py_gen: move TLVs into their own module
java_gen: support TLVs
loci: support bsn_gentable
loci: add max_entries member to of_bsn_gentable_desc_stats_entry
openflow_input: add more TLVs for the ARP agent
openflow_input: clarify bsn_gentable
Merge into master from pull request #165:
bsn_gentable extension (https://github.com/floodlight/loxigen/pull/165)