Merge #1462: Refactor and cleanup of blockchaininterface and related

3fc74fb Refactor and cleanup of blockchaininterface and related (Kristaps Kaupe)

Pull request description:

  Summary of changes:

  * Add typehints to `jmclient/jmclient/blockchaininterface.py` and `jmclient/jmclient/jsonrpc.py`;
  * Move all methods called by external code to `BlockchainInterface` base class or add abstract methods there;
  * <del>Remove broken `ElectrumWalletInterface` (we can re-introduce it later from history if somebody wants to fix it);</del><ins> (done in #1477)</ins>
  * 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.

  #1460 was part of this, but did separate PR for that one, as it is simpler to review small refactoring changes.

  This should make it more easy to: 1) write new code that uses blockchaininterface, 2) write new alternative implementations of `BlockchainInterface`.

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Tree-SHA512: 7699de0419c1006ff3c6ac5da7d26055b8e3508c424b5bdd3117fea575d30ab21e8617b2d50ae7c8b2e76997d6f4b14781bb56fcdad4efc081d533a713666a31