This PR corresponds to:
https://github.com/freqtrade/freqtrade/issues/1377#issue-386200394
in understanfing that pair Ticker is mostly statistics, but on the other side, create_trade/execute_buy.
It resolves problem with some exchanges (BitMex) where ticker structure returned by ccxt does not contain bid and ask values.
1. On exchanges like Bitmex, set use_order_book: true for buys. FT won't request ticker and will use data from order book only.
2. On exchanges where order book is not available, set use_order_book: false, ticker data (including ask/last balance logic) will be used.
3. On other exchanges, either approach may be used in the config.
Performance: current implementation fetches ticker every time even if order book data will be later used. With this change it's eliminated.
Comparison of order book rate and ticker rate is removed (in order to split fetching order book and ticker completely in execute_buy), so some tests that touch this code may require adjustments.
e8ef36fb6e