Add tracing to the transaction manager

This commit adds distributed tracing support to the WAL transaction manager. Each transaction spans three stages: preparation, committing (to be logged to wAL), and application. The first two stages could be scoped by RPCs. The last stage is handled by a long-running goroutine. Hence, we ignore the last stage at the moment.

Result

Enabling Jaeger in the local environment, we gather the following results:

Read-only transaction	Mutating transaction