Patch to avoid mailboxesdb corruption on concurrent renames

[Bron Gondwana]

I've put a header on the patch describing the bug.  Basically,
the result code from mailbox_open_locked() wasn't being tested
sufficiently, and hence the new mailbox name would be created
in mailboxes.db even though the files were no longer available
to be copied - causing sync bailouts and IOERRORs and all sorts
of fun.

What causes this?  Clients that send a RENAME or DELETE event
and then you do something else in the GUI and they open a
_separate_ connection which tries to do something else to the
source folder.

I think it is our only remaining source of bailouts!  It
requires a reconstruct to fix when it happens.

Patch attached, and the perl script I used to confirm that it
existed and confirm that this patch fixed it.

Regards,

Bron ( P.S. isn't it about time for a 2.3.12?  I'm getting sick
       of posting skiplist patches to people running the
       lastest and having issues! )
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