Make mempool-removed tracking much more explicit #5433

Based on #5267. The conflicted transaction tracking during mempool remove is actually critical for balance stability, as @dgenr8 and @morcos pointed out previously...but for a very subtle reason. When we called SyncTransaction, it was doing a ton of work...just to uncache the balance. Instead, this pull replaces the SyncTransaction call with a new wallet signal, which only provides the transaction hash, making the removed tracking a bit lighter weight.

Also, it explains what is actually happening and adds a python test-case to test it out, instead of it being entirely opaque and marginally fragile.

Looks good, I plan to test. If you can call the new signal from txmempool directly during remove, then you can also get rid of the to-be-removed list.

And I'll just mention that the wallet relying on the volatile and policy-dependent mempool for conflict detection is something that could be improved (in the wallet).

I did some testing with the mempool-removed tracking part of this pull, and the caching of credit balances now seems to be properly updated.

After all the travis round-tripping, it appears the LogPrints in the DEBUG_LOCKORDER routines were killing the ability to restart bitcoind, so those are now gone and this is reviewable.

Untested ACK.

Needs rebase

Rebased.

ut ACK

concept ACK

Also fixed the test cases after rebase.

@TheBlueMatt Needs rebase

@TheBlueMatt not worth rebasing?

Meh.