Rewrite and simplify name_list a bit.

Rework name_list, removing unnecessary variables and cleaning up the
code.  It also doesn't load full transactions from disk anymore, since
they are already in the wallet.  It has to access the txindex, though.
Further tweak name_list.

Load also the txindex from disk only right before it is actually used.
This skips the disk access entirely if we don't have a name tx at hand.
Merge pull request #61 from domob1812/name-list

Name list improvements
Port over initial UTXO DB changes from Namecoin.

Port over the initial commit to implement an UTXO DB from Namecoin.  It
can keep track of the UTXO set in ConnectInputs, but not yet in
DisconnectInputs.  Also includes listsinceblock.
Update the UTXO set in DisconnectInputs.

Also handle the UTXO set in DisconnectInputs.  Not yet really tested.
Remove duplicate listsinceblock, error from merge.
Fix rescanning for Huntercoin.

Fix differences between NMC and HUC in UTXO rescanning.  In particular,
add also the genesis block coinbase and process both block.vtx and
block.vgametx.