Migrate MiniNode to Zcash
This enables various RPC tests that use it (most of them in the extended test suite) to properly test Zcash code.
The PR also fixes bugs in the BIP65 and BIP66 tests that were both masking and masked by the un-migrated MiniNode.
The Python module `pyblake2` is now a requirement for the RPC tests.
Part of #2530.