Add a method AssertRead to insert a key-version to the TX read-set

[Yoav Tock]

Some times there is a slow changing key-value that many following TXs depend upon. Instead of reading it on every TX, we can read it once, and then inject the key and version into the read set of future TXs, just using the read value. This saves a read on every future TX, saves RTT to the server.

[Yoav Tock]

Method signature:
AssertRead(db, key, version) error

To add to the tx context