Calling getblocktemplate without the segwit rule specified is most
likely a client error, since it results in lower fees for the miner.
Prevent this client error by failing getblocktemplate if called without
the segwit rule specified.
This trivial change adds the "override" keyword to some methods of
subclasses meant to override interface methods. This ensures that any
future change to the interface' method signatures which are not correctly
mirrored in the subclass will break at compile time with a clear error message,
rather than fail at runtime (which is harder to debug).
After a recent bug discovered in callback ordering in MainSignals,
this test checks invariants in ordering of
BlockConnected / BlockDisconnected / UpdatedChainTip signals
