[Raft] Add a transaction janitor

[luluz66]

Related issues:
https://github.com/buildbuddy-io/buildbuddy-internal/issues/3279

• Add a proto TxnRecord and store it in meta range at different stages of
  RunTxn.
• Add a transaction janitor process to read the txn record and help get the
  stuck transactions unstuck.

[tylerwilliams]

overall this looks good -- just a few comments!

[tylerwilliams]

it's kind of messy how DeleteTxnRecord and WriteTxnRecord are in the sender but called from the janitor. Could all of these txn coordinator methods be moved to a txncoordinator file, along with the janitor code?

[luluz66]

it's kind of messy how DeleteTxnRecord and WriteTxnRecord are in the sender but called from the janitor. Could all of these txn coordinator methods be moved to a txncoordinator file, along with the janitor code?
I moved the txn related methods including RunTxn from sender to txn.go.

[tylerwilliams]

nit: just call this Coordinator