Emulate methods to accept both hex and base64 encoded boc

[aleksej-paschenko]

• Emulate methods to accept both hex and base64 encoded boc
• Fix mutex usage
• Implement mempool subscription with additional emulation results.
  How to subscribe to mempool with emulation results:
  /v2/sse/mempool?accounts= for sse
  "subscribe_mempool" with params=["accounts="] for websocket.