Address lookup removed by accident

Infernoman

Also cap the allocation for the leveldb-specific cache for the UTXO set
to 8MiB.
This avoids that the extra cache memory goes to the much less effective
leveldb cache instead of our application-level cache.

If the mempool is not completely full, treat the difference between
the maximum size and the actual usage as available for the coin cache.

This also changes the early flush trigger from (usage > 0.9 * space)
to (usage > 0.9 * space && usage > space - 100MB). This means we're not
permanently leaving 10% of the space unused when the space is large.

CDarkSendSigner->CLegacySigner
darkSendSigner->legacySigner

Make sure we're checking disk space for block index writes and allow for pruning to happen before chainstate writes.