Currently, there are a bunch of hacks in TxnCoordSender with respect to contexts and cancellation. In particular, since TxnCoordSender must implement the Sender interface, it has no way of obtaining a reasonable transaction-scoped context. See https://github.com/cockroachdb/cockroach/blob/dda3ee2/pkg/kv/txn_coord_sender.go#L603:L607.

A further consequence of the above is that TxnCoordSender cannot depend on the semantic correctness of the context it is given if that context is not cancellable. See https://github.com/cockroachdb/cockroach/blob/dda3ee2/pkg/kv/txn_coord_sender.go#L729:L742.

We should hoist TxnCoordSender into the client.DB's Txn() method, which already takes a transaction-lifetime context, and remove the hacks described above in the process.

This is a pretty meaty task; good for someone looking to get acquainted with kv.

cc @andreimatei @paperstreet @bdarnell @petermattis @tschottdorf