Tracking: Support invalidating and reconsidering blocks in the NonFinalizedState via RPC methods

[arya2]

Motivation

This would be useful for testing forks and chain reorganizations, it was mentioned in the forums as a requirement for dex support and in the Regtest tracking issue.

See this comment for more information about the motivation.

Design

Add invalidate()/reconsider_block() methods to the NonFinalizedState, accept a new channel in write_blocks_from_channels() running in the the block write task (or refactor the existing channels into a single channel receiving an enum), add StateService Request variants for invalidating/reconsidering blocks, and add the invalidate()/reconsiderblock() RPC methods that call the StateService with the new Request variants.

Implementation Plan:

See Sub-issues below