Split out methods to every module, apart from 'help' and 'stop' which
are implemented in rpcserver.cpp itself.
- This makes it easier to add or remove RPC commands - no longer everything that includes
rpcserver.h has to be rebuilt when there's a change there.
- Cleans up `rpc/server.h` by getting rid of the huge cluttered list of function definitions.
- Removes most of the bitcoin-specific code from rpcserver.cpp and .h.
Continues #7307 for the non-wallet.
Creates the generatetoaddress rpc which is virtually identical to the generate rpc except that it takes an argument for the address to mine to. It does not rely on wallet functionality.
The mining code shared by generate and generatetoaddress has been moved to another method to reduce duplication.
