Make 'compactMinRetain' and 'compactInterval' configurable

[sukumargaonkar]

Usecase:
Planning to use kine+postgres to store mostly immutable data (very few updates though out its lifetime).

Current scenario:
To keep db size low, kine supports compaction and cleans up old revisions as per below conditions (as described here)

• versions older than compactInterval (default compactInterval 5mins)
• Always retain the last 1000 revisions (default compactMinRetain is 1000)

The 'compactInterval' and 'compactMinRetain' both are constants and thus cannot be configures

Feature Request:
Make 'compactMinRetain' configurable. When dealing with rarely updating data, setting the 'compactMinRetain' value low would help keep db size low