Post-0.21: Separate wallet and node

[apoelstra]

In https://github.com/bitcoin/bitcoin/pull/16367 the various Bitcoin Core were largely separated. In the 0.21 rebase of this PR I added libbitcoin_common_a_SOURCES and libbitcoin_server_a_SOURCES to libbitcoin_wallet_a_SOURCES which partially undid this separation.

The reason for this was that the blocksigning RPCs, while being part of the wallet, need access to the node. Similarly the pegin RPCs also look at the current chain state and even accesses the Bitcoin RPC.

Properly separating these things may be nontrivial. Blocksigning we should likely just move to a separate utility rather than being part of the wallet. Pegin claim management will take more thought.