8074597e mailbox.c: release cache files when locking index

[Дилян Палаузов]

Hello,

I have the problem that on 3.0 sometimes the cyrus.cache gets truncated to 4 bytes (usually but not always by
cyr_expire) and then it takes on Inbox very, very much IO and time to reconstruct cyrus.cache, and after a while
cyrus.cache gets trucated again.

Does this commit https://github.com/cyrusimap/cyrus-imapd/commit/e8074597e84cfb62cc fix the problem, is it useful for
3.0, does somebody see in syslog similar problems?  What problem does this commit solve?

Greetings
  Дилян