bittrex leftover bug fix ccxt#3329

[ci skip]

bittrex optional price precision by quote currency code fix ccxt#3307

[ci skip]

optional okex default behaviour fix ccxt#3316

[ci skip]

kucoin fetchTickers marketId / symbol fix ccxt#3328

[ci skip]

okex CAN → Content and AD Network fix ccxt#3327

[ci skip]

coinex: fetchMarkets: properly set market['active'] flag

[ci skip]

minor fix in fcoin parseTicker with market

[ci skip]

a better typecheck for a defined/undefined market everywhere

[ci skip]

zb throw ExchangeNotAvailable on "Busy Server"

[ci skip]

kucoin calculate_fee fix ccxt#3261

[ci skip]