Bittrex: Update API v1.1 to v3 and add websocket support (#646)

* Update Bittrex API from v1.1 to v3

V1.1 has been retired as of 9/30/2020

- Update REST to V3
- Add initial websocket support

* Bittrex update - enable websockets in testdata config file

* Update Bittrex - add Websocket capability to docs

* Update Bittrex connector

- AppVeyor warnings
- Update tests
- Generate documentation
- Fix nits

* Update Bittrex - add websocket order processing

* Update Bittrex connector

* Bittrex connector - fix ineffectual err assignment

* Fix nits

* Orderbook synchronization

* Remove redundant nil

* Log WS fetch orderbook message as debug message instead of as warning

* Update after rebase

* Add tests

* Add allowed candle interval values

* Replace literals with declared constants

* Replace variable name 'request' with 'req'

* Add check and update for deprecated REST URL

* Nits and some cleaning up

* Change ParseInt bit size to 64

* [FIX] Remove several shadow declarations

* Do not export constructTicker

* Remove parseTime()

* Update GetHistoricCandles()

* [FIX] Address gocritic nits

* [FIX] Address gocritic nits

* Use SendMessageReturnResponse() instead of local map

* Rate limit subscribing and unsubscribing

* [FIX] use go routine for subscribing and unsubscribing

* [FIX] Set correct index for map

* [FIX] Address unused vars, literals, time format

* Adjusted timing when subscribing to many order books

* Cache partial updates to tickers instead of calling REST function

* [FIX] Update sequence nr when multiple updates are queued

* Address golint issues

* Fix nits

docs/ADD_NEW_EXCHANGE.md

@@ -202,7 +202,7 @@ Yes means supported, No means not yet implemented and NA means protocol unsuppor
 | Bithumb | Yes  | NA       | NA  |
 | BitMEX | Yes | Yes | NA |
 | Bitstamp | Yes  | Yes       | No  |
-| Bittrex | Yes | No | NA |
+| Bittrex | Yes | Yes | NA |
 | BTCMarkets | Yes | No       | NA  |
 | BTSE | Yes | Yes | NA |
 | COINUT | Yes | Yes | NA |