Add support for bitcoinRpcTimeout

[dhuseby]

While testing ion-bitcoin I slowed down my bitcoind so that RPC responses were really slow and the BitcoinClient quickly timed out and ion-bitcoin that uses sidetree died.

This is a feature request to add a new "bitcoinRpcTimeout" config value here and then modify the BitcoinClient constructor to accept the value passed to it from the BitcoinProcessor. The value would then get used here when calculating the timeout for the RPC request.

This would allow for the tuning of RPC timeouts to handle cases where bitcoind is running on a slow machine or a machine with slow storage.