Add ElectrumX transaction merkle endpoints

[JeremyZeng77]

Issue

Fixes #9.

Summary

• adds GET /electrumx/tx/merkle/:txid/:height
• adds POST /electrumx/tx/merkle for bulk txid/height requests
• documents both endpoints with apidoc comments
• adds unit mocks, unit tests, and integration coverage for the new handlers

Verification

• git diff --check
• node --check src/routes/v5/electrumx.js
• node --check test/v5/a01-electrumx.js
• node --check test/v5/integration/electrumx.js
• node .\node_modules\standard\bin\cmd.js --env mocha src\routes\v5\electrumx.js test\v5\a01-electrumx.js test\v5\integration\electrumx.js test\v5\mocks\electrumx-mock.js
• NETWORK=mainnet TEST=unit node .\node_modules\mocha\bin\mocha --exit --timeout 60000 --grep "(#getTransactionMerkle|#transactionMerkleBulk)" test\v5\a01-electrumx.js: 11 passing
• NETWORK=mainnet TEST=unit node .\node_modules\mocha\bin\mocha --exit --timeout 60000 test\v5\a01-electrumx.js: 98 passing

Payout

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