blockchain: Add UtxoCache Initialize tests.

[rstaudt2]

This is rebased on #2591.

This adds additional tests around utxo cache initialization to ensure that it recovers in various scenarios that are hard to simulate or hit with manual testing. The changes are as follows:

• blockchain: Make InitUtxoCache a UtxoCache method.
  • This changes the InitUtxoCache method on the BlockChain type to a Initialize method on the UtxoCache type instead. This simplifies providing alternative implementations for testing since the Initialize method deals with internal fields of the UtxoCache type.
• blockchain: Add UtxoCacher interface.
  • This adds a UtxoCacher interface so that alternative utxo cache implementations can be provided. In particular, this will be used to provide a mock implementation for testing in order to more easily simulate various scenarios.
  • In addition to introducing the UtxoCacher interface, this updates BlockChain and UtxoViewpoint to use the interface rather than the concrete type.
• blockchain: Add UtxoCache Initialize tests.
  • This adds tests for the UtxoCache Initialize method. These tests include recovery scenarios that are particularly hard to simulate to ensure that those code paths are not broken in the future.
  • In order to test recovery scenarios, this introduces a test utxo cache that allows for toggling flushing on and off.
  • Additionally, this adds an ExpectUtxoSetState method to chaingenHarness that allows for easily validating the last flushed block for the utxo set.
• blockchain: Add TestShutdownUtxoCache tests.

[davecgh]

It's great to see more tests around this to ensure it's handling the edge cases properly.

I noticed a few minor things that I've noted inline.