Implement Simplified Papment Verification

[jimmysong]

1. Add calls necessary for SPV verification to electrum.py's EnhancedBlockchainState and chroma.py's ChromaBlockchainState.
2. Add verify_merkle form https://github.com/spesmilo/electrum/blob/master/lib/verifier.py to both files mentioned in (1)
3. Subclass BaseTxDb for a class called VerifiedTxDb. Add a method identify_tx_status which uses the verify_merkle method in the blockchain_state object in model to conclusively determine the actual TX status instead of having to trust anybody.