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.
Some exchanges (BitMEX) return integer values for Volume field. And sometimes even for OHLC -- same, on BitMEX, since price decrease is 0.5. TA-LIB functions assume floats and fail with exception.
Of course, this can be fixed (converted) in ccxt for particular exchange, but TA-LIB will still fail for exchanges for that such a conversion is not implemented in ccxt code. So let's make perform this conversion here in order to be sure our strategy will not crash on a new exchange.
Output stake_amount as it is defined in the config (it may by int) instead of float. In order to avoid unnecessary questions where and why it was converted to float and changed in the last digit while it should be integer for the exchange...
Other small cosmetic improvements to logging in freqtradebot.py
