exchanges: API coverage improvements and helper functions (#681)

* improved functions and new helper functions

* bitfinex margin info func

* small rate change

* rate changes

* adding some currencies for margin funding translation

* adding index candles

* added test

* slight improvement in params

* time func

* orderbook helper avgprice func

* broken test + removing some tlogs and prints

* adding test cases

* error fix

* remove unused

* another unused

* shazbert changes

* wip

* bitfinex func and more nits

* final shazzy nits

* most shazzy nits

* few prior requested changes

* shazbert nits final WIP

* shazbert changes

* minor linter issue

* unused val

* glorious changes

* more verbositiy improvements

* quick changes

* unused remaining amount oops

* thrasher changes

* reverting changes that were only for testing purposes and bymistake pushed up

* bfx shadow dec + huobi fetch tradable pairs formatted so as to return config format for ease of comparison and requests

* more linters

* glorious final nits wip

* formatting tradable pairs for different asset types + remove println

* glorious changes

testdata/http_mock/binance/binance.json

@@ -255890,6 +255890,18 @@
     }
    ]
   },
+  "/fapi/v1/time": {
+   "GET": [
+    {
+     "data": {
+      "serverTime": 1620099477465
+     },
+     "queryString": "",
+     "bodyParams": "",
+     "headers": {}
+    }
+   ]
+  },
   "/fapi/v1/trades": {
    "GET": [
     {