Removes RagManager dependency on TxManager since all lock requests needs
to pass in the transaction explicitly. There's now no such thing as trying
to figure out the transaction late by using the transaction manager.

This makes things more explicit, less entangled and easier to reason
about.
Merge pull request #514 from tinwelint/detangle-rag-and-tx-mgr

Removes RagManager dependency on TxManager