Auto merge of #3520 - Eirik0:3327-sign-offline, r=bitcartel

Auto merge of #3520 - Eirik0:3327-sign-offline, r=bitcartel

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.