[PATCH 00 of 12] filecache fixes

[Idan Kamara]

After some extensive debugging of recent filecache issues with convert*, I believe
this series will finally put those and future issues to rest.

I've invested quite a bit in making the commits clear and descriptive because this
area isn't easy to get into.

Due to the size of the series, pushing it to stable might be risky, even so I've
developed it on top of stable (also ran the full test suite on every commit in the series).

* http://mercurial.markmail.org/thread/lj2ynxnslkvfrhul
  http://bz.selenic.com/show_bug.cgi?id=3693
  http://bz.selenic.com/show_bug.cgi?id=3335