Wallet code should use interfaces::Chain and not directly access to node state.
Add a g_rpc_chain replacement global for wallet code to use, and move
g_rpc_node definition to a libbitcoin_server source file so there are link
errors if wallet code tries to access it.
https://github.com/bitcoin/bitcoin/pull/16839/commits/362ded410b8cb1104b7ef31ff8488fec4824a7d5
Depends on D6418
Concludes backport of Core PR16839