Create mx_ops.sync operation. Refactor compress to use the mx_ops.sync.

Create mx_ops.sync operation.  Refactor compress to use the mx_ops.sync.

Change compress.sync_mailbox() to lock the compressed mailbox around
both the tempfile sync and compress operations.  This will prevent
changes made inbetween the two syncs from being overwritten.

Thanks to Damien Riegel for his original patch refactoring
mx_ops.sync, which this patch is partially based upon.