Update pool options during its life time

[WyriHaximus]

A use case for this is scaling the amount of workers up and down when the load of the systems permit it.

• [x] Update pool interface
• [x] Update manager interface
• [x] Update fixed pool
• [x] Update flexible pool
• [x] Update fixed manager
• [x] Update flexible manager
• [x] Update documentation
• [x] Add example