Did some more exploration and perf tuning and thinking on caching
databases, and am pretty sure I know how I want to implement it. Will be
several stages, starting with using it for generating views, and ending(?)
with using it for direct mode file mappings.

Not sure I'm ready to dive into that yet, so instead spent the rest of the
day working on small bugfixes and improvemnts. Only two significant ones..

Made the webapp use a constant time string comparison (from `securemem`)
to check its auth token is valid. Could avoid a potential timing attack to
guess the auth token, although that is theoretical. Just best practice to
do this.

Seems that openssh 6.5p1 had another hidden surprise (in addition to
its now-fixed bug in handing hostnames in `.ssh/config`) -- it broke
the method git-annex was using for stopping a cached ssh connection,
which led to some timeouts for failing DNS lookups. If you see git-annex
seeming to stall for a few seconds at startup/shutdown, that's maybe why
(--debug will tell for sure). Seem to have found a workaround that avoids
the problem.