Turn a wfDebug call in MessageCache::get into a wfDebugLog call

It's a very hot code path, so it makes sense to be able to dive into it in
detail, and having a discrete log bucket makes that easier to do.

Change-Id: I4b547e97711b9e45a99f7b747d785690c70883ec
Merge "Turn a wfDebug call in MessageCache::get into a wfDebugLog call"