the basic idea is replace txIndex key with a pair (username,height).
height = -1 for the most up-to-date key, otherwise height = last block where previous key was valid.
by checking maxHeight and iterating backwards we can easily find the key to validate data from any given block number.
The new class is accessed via the Params() method and holds
most things that vary between main, test and regtest networks.
The regtest mode has two purposes, one is to run the
bitcoind/bitcoinj comparison tool which compares two separate
implementations of the Bitcoin protocol looking for divergence.
The other is that when run, you get a local node which can mine
a single block instantly, which is highly convenient for testing
apps during development as there's no need to wait 10 minutes for
a block on the testnet.
