[DO-NOT-MERGE] txpool: streamline initialisation

[taratorio]

PR IS IN DRAFT (but want to run CI)

Attempt to simplify how we initialise and run theTxPool.

Observations:

1. Currently the package exports many sub-components to backend.go such as Fetch, Sender, PoolDB, newTxs chan txpool.Announcements. I see these all as internal components of the TxPool that backend.go and other initialisers should not care about. In reality, the only things backend.go cares about are the TxPool and the TxPoolGrpcServer.
2. Additionally, logic about starting and closing the tx pool leaks to backend.go (ie the creator of tx pool needs to know what sub-components need to be started and disposed).

This PR addresses the above by:

1. Moving the p2p fetcher, sender and pool db as internal attributes of TxPool.
2. Introducing the Run(ctx) pattern that we are using in Astrid and Shutter where each runnable component is responsible for starting its own background goroutines, loops, listeners, etc. and also is responsible for shutting them all down by being ctx-aware (ie when ctx is cancelled all sub components gets disposed of/shutdown)