Add support to configure requests pool options

[froggleston]

This PR adds support to control the number and size of pooled connections for the requests HTTPAdapter. If you're using quite a lot of bots within FTUI, the pool can become saturated and warnings will appear in the terminal. Increasing the pool_connections and pool_maxsize properties can help with this.

These two options:

• can now be supplied in the FTUI config yaml or on the command line if using a freqtrade config file
• default to higher numbers than the requests library/ft rest client defaults (10 and 10, respectively are now 20 and 10)