Fix signing raw transactions with unsynced offline nodes
This PR address the issue in two different ways:
- In `signrawtransaction` we determine the consensus branch ID (which we then later use to construct the transaction) using the chain height. We now also consider the `APPROX_RELEASE_HEIGHT` as this is a better estimation than 0 for the height of the chain if we are unsynced. (This in and of itself solves the Overwinter signing issue).
- We have added an additional parameter to `signrawtransaction` to allow manually overriding the consensus branch ID that zcashd determines we are on. This allows users to work around corner cases where the first strategy is still insufficient.
Closes#3327.
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.
