Create bitcoinj-based Bitcoin (SPV) Wallet server

[msgilligan]

See bitcoinj/bitcoinj#895

[msgilligan]

To keep things simple I'd like to focus initially on functions that can be implemented without a BlockStore, using only the PeerGroup component of bitcoinj. But if you want to contribute other functions, the help is more than welcome.

Update: this has changed there is now BlockStore and some basic SPV Wallet functionality using bitcoinj's WalletAppKit.

[msgilligan]

The bitcoinj-daemon module was created for this purpose and currently serves a "very small subset" of Bitcoin RPCs.

[msgilligan]

The cj-btc-daemon-mn submodule partially implements a useful subset of Bitcoin Core's Blockchain RPCs and natively compiles with GraalVM native-image.

With a little bit of refactoring and adding some documentation (in the v0.6.0 release), we can close this issue and open new issues and enchantment requests to drive this submodule forward. (At some point in the future maybe even migrate to its own repo)

[msgilligan]

There cj-btc-daemon-mn module was renamed to cj-btc-daemon and is now cj-btc-walletd (as of PR #250)

I have retitled this issue to reflect that the server is a Wallet Server (as opposed to a full-node)

I will also open a new Issue for creating an Electrum server.