lib/log: migrate to github.com/sourcegraph/log

Created by: bobheadxi

We are migrating lib/log to https://github.com/sourcegraph/log in the hopes of easing adoption for other services that Sourcegraph depends on, such as Zoekt (https://github.com/sourcegraph/zoekt/pull/355#issuecomment-1150119531)

Test plan

All tests still pass