Cleanup logging handlers

We were leaking logging handlers which resulted in slowing down test
runners as they ran. This got worse as more tests ran in a single
process so individual tests run slower with less contention in the test
suite.

Fix this by cleaning up the handlers at the end of each job. Local
testing shows that this doesn't fix all the full test suite slowness but
reduced slowest test runtime by a third.

Change-Id: Iaa4b44cd98f7c874a978aef5fcb290fccf105650
1 file changed