This PR adds cleanwallettransactions RPC call to VerusCoin which will work with VerusCoin itself and should work with any PBaaS chains. It makes the wallet operation faster and dramatically reduces response time of certain RPC calls.

The RPC call removes old transactions from wallet.dat until the oldest UTXO. In order to clean past the oldest utxo, you have to spend that utxo or more to get the desired effect.

This does not reduce the size of wallet.dat but it removes the txs and the daemon can write new txs on those empty spaces.

Here's an example output of the RPC call in vrsctest network:

./vrsctest cleanwallettransactions
{
  "total_transactions": 29476,
  "remaining_transactions": 11092,
  "removed_transactions": 18384
}

@miketout please edit as necessary.