adding txlog
update gitignore
txlog and tests
txlog now committed via rangetxn mechanism, which increments rangeversion
flip write_api to exceptions, so that RAII txn objects can abort cleanly
additional unittests for versioning; and a fix for vertex-tag key values not being versioned correctly
update write_api to include an "id", which will be the proposer id used in PAXOS; this will be used to ensure startup in stored is current before, and all learned requests are >= those already replayed before retiring the replay
adding sequence number field, and changing proposal_num back to 64bit
Merge branch 'add_txlog'