Currently the dirlist ignores the counts provided by the dirInfo.
However some of the workers can actually provide accurate counts much quicker
than if we count the flags.
Eventually we will also want to enable displaying counts for background folders,
where the brute force counting won't work as none of the headers are fetched yet.
This commit models it in an opt-in manner, if the flag isn't set then we still
count the messages manually.
66b68f35b3