lib/log: add IncreaseLevel to make a quieter child logger
Created by: bobheadxi
Addresses this use case: https://github.com/sourcegraph/sourcegraph/commit/33eb4af6394c7ebc7fcf7ac3a851336d77059952#diff-131e98c60f40a8485b5d19a455d41ddb05c97f2dbda8ded9b2094abb8af60668L146
Now loggers can use IncreaseLevel
to indicate a component of the codebase that they are opting to make quieter.
Test plan
n/a, it's a very thin wrapper over existing Zap APIs