Fix: Fix Stats and Utxo cache

[junderw]

Fixes #11

At the very least it fixes the more embarrassing ones.

1. Negative balance
2. Non-zero balance even though number of funds and spends txos are equal etc.

Also, comparing the confirmed_height in the history key to the blockid height (from the local best header chain) is a more efficient way to weed out the re-confirmed-in-separate-block issue.
If a re-org happens and the same TX is in the same height block, then it will perfectly match the existing key (and won't duplicate)

[mononaut]

utACK @ a824fd8e

I haven't been able to find any bad stats locally, or reproduce the issue in regtest, so I couldn't test the actual fix.

but the code looks good, and it doesn't seem to break anything.

[softsimon]

utACK