Refactor and cleanup of blockchaininterface and related

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;
* Remove broken ElectrumWalletInterface (we can re-introduce it later
  from history if somebody wants to fix it);
* 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.