Add separate bitcoin-rpc client

Rework of #3072. This just adds the new executable.
I gave up on the code movements to remove the dependency on leveldb/BDB as things are just too entangled, see #3072 for details. Hopefully this will also make the pull tester happier. And this is easier to review, too.

This adds an executable `bitcoin-rpc` that only serves as a Bitcoin RPC client.
The commit does not remove RPC functionality from the `bitcoind` yet, this functionality should be deprecated but is left for a later version to give users some time to switch.