deal with read-your-own-writes for txns and snapshots

[dotnwat]

7dc2521

see rocksdb transaction interface

[dotnwat]

turns out the issue i was observing was actually because the read immediately after the txn committed was not taking the db lock when copying the root pointer. otherwise the original version with the commit referenced in this issue description would have worked. in either case, we should consider how best to handle this and look at rocksdb for guidance. 7dc2521 and dc9afac