Another tweak for 'SL-15547: Viewer hung while looking for a file in cache' - this time based on Henri's suggestion in this discussion: https://bitbucket.org/lindenlab/viewer/commits/e28c1b46e9944f0215a13cab8ee7dded88d7fc90#comment-10537114

Additional speculative fix for SL-15547: Viewer hung while looking for a file in the cache. I am still unable to reproduce this problem, although others have so I am trying a more aggressive approach for them to test.  I tried this as part of last speculative fix but saw what I thought were scary side effects in the log/console.  Turns out, those are there in the release viewer so probably unrelated to this change.

Speculative fix for SL-15547: Viewer hung while looking for a file in the cache - since I am unable to repro, this might be enough

Also add Ansariel's explanation for why interaction through the filesystem
itself should be safe.

Ansariel kindly offered their patch to help mitigate this round of file system issues - taken from https://vcs.firestormviewer.org/phoenix-firestorm/changeset/104a8600946be01e2de44d10ad069ba854272d1f

This set of changes reverts the merge with master (git revert c83e740e) and results in a version of the DRTVWR-519 that matches what was presemt before it was deployed as a release viewer *plus* 3 small fixes from Maxim (See commits).  This branch can now be used for additional fixes before eventually being used to release D-519 as normal

This reverts commit b1ab29ee.