Summary of changes:
* Add typehints to blockchaininterface.py and jsonrpc.py;
* Move all methods called by external code to BlockchainInterface base
class or add abstract methods there;
* More dummy abstract method overrides for DummyBlockchainInterface (for
tests);
* Alphabetical ordering of imports and other minor stuff;
* Behaviour change - previously fee estimation would fail if it could
not get mempoolminfee, now will go with default 10 sat/vB.
Co-authored-by: Pulp <51127079+PulpCattel@users.noreply.github.com>