This removes the attempt at magic (predicting future) component
called the fee-estimator.
The direct effect is that all transactions have as a mempool acceptance
the same minimum fee. Regardless of how full the mempool or blocks were.
This mempool-acceptance minimum fee is 0 sat/byte. (aka free)
Notice the node-wide free-tx accept-limiter is going to avoid us getting
overloaded.
The new UnspentOutputDatabase classes are only very loosly a database, we
purely register and store unspent outputs there. But unline a DB we don't
allow modification (just insert and delete).
This replaces the coin-db (which was based on leveldb) and as first goal
this gives us a higher level of stability. The level-database was known
to give corruption issues.
Notice that users will need to do a manual reindex on first update.
TomZ