Evict coinbase spends from memory pool during re-org

This is an alternative version of #5267 

It works by having DisconnectTip() lookup the block at height current-COINBASE_MATURITY, and then removing any transactions spending that coinbase (recursively) from the memory pool.

Since re-orgs happen by repeated calls to DisconnectTip(), this will keep the memory pool always consistent, even in the middle of a re-org.

Builds on #5316 

Includes a new regression test.
To mimic the old behavior, you actually need to push_back here. Though, again, I dont think this code is even necessary.