[Snapshots] Automatic configuration tuning

[eminano]

Is your feature request related to a problem? Please describe.
Currently the snapshot configuration can be tweaked to improve performance. However, it can be hard to know which values to use for each individual case.

Describe the solution you'd like
Have a way of automatically tuning the snapshot configuration to have optimal performance during the snapshot process.

Describe alternatives you've considered

• Improve documentation around configuration tuning
• Automatically tuning the configuration parameters during the pgstream snapshot command (especially for large source databases)
• Add a dedicated command pgstream snapshot tune that outputs the configuration after performing test/analysis of the source/target database.