#4812 Fix/hitbtc trading pairs parsing

A description of the changes proposed in the pull request: Change in HitBTC connector to keep a list of available tokens and use it to translate tokens from exchange format to client format back and forth. Since the connector is a spot connector the new logic excludes all perpetual tokens from the list.

Tests performed by the developer: I have added unit tests for all the new functionality related to the population of the token list and the new functions to translate token formats. I have not added tests for the connector functions that consume the translation methods because we need to send this fix to production with the next release. Also in the near future we will be working on replacing websockets library for aiohttp in HItBTC, and we will add all the required tests when tackling that task.

Tips for QA testing: Please test the connector can create and cancel orders correctly, and the trade fills are processed without issues. I could not test the connector myself because the support HitBTC account has no founds.

Fixes #4750

https://github.com/CoinAlpha/hummingbot/pull/4812