Flushes the blktree/ and coins/ databases, and reindexes the
block chain files, as if their contents was loaded via -loadblock.
Based on earlier work by Jeff Garzik.
Split off CBlockTreeDB and CCoinsViewDB into txdb-*.{cpp,h} files,
implemented by either LevelDB or BDB.
Based on code from earlier commits by Mike Hearn in his leveldb
branch.
Pieter Wuille