Object Storage alpha lands on master

[Bron Gondwana]

I've pushed the objectstorage code based on Raymond and Jean-Francois's work to master.  It's almost exactly what was on Raymond's branch with just a couple of modifications.

1) updated for some API changes I'd made to archiving in append.c
2) fixed compilation under warn-unused-variables
3) normalised logic between append_fromstage and append_copy for handling objectstorage errors by creating non-ARCHIVED files.
4) fixed file copy in mailbox_archive to work the same way as the file moves, where it correctly passes the source/destination file to objectstorage and gets the file back, so unarchive works.

WARNING WARNING WARNING: while I've tested that normal operation and normal archive file operation (non-objectstorage) works correctly with Cassandane and FastMail experimental branch, I haven't done more than rudimentary testing with the dummy objectstorage.

I'm almost positive that reconstruct won't work correctly with object storage turned on right now.

This is still very much work in progress - but putting it on master means that the code will be maintained and updated as other API changes are made.

Thanks Raymond and Jean-Francois for all your work on this.  I hope it gets more testing and use.

Cheers,

Bron.

-- 
  Bron Gondwana
  brong at fastmail.fm