Build Blockbook mock server

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We will need a Blockbook mock server which can operate similar to a regtest server for our QA/integration test suite. We had an insight spike at #28 which would be a good pattern to start with.

Mock should be able to:

• create funds for any address
• simulate new transactions found
• simulate any number of new found blocks
• respond with "reasonable outputs" for all endpoints (also considering simulated values)

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We might consider reviewing how blockbook handles their integration tests and finding out how we can produce their verifications for our purposes: https://github.com/trezor/blockbook/blob/master/tests/rpc/rpc.go

• (👍) I would prefer that we had some live server rather than some mock software that risks becoming obsolete and using their integration tests might shed insight on using some mock which is closer to reality.
• (👎) On the other hand, blockbook is under heavy development and we risk our test suite rotting if their library migrates and we do not move swiftly with it.