lib-storage: Add mail_sort_max_read_count setting. This controls how many slow mail accesses sorting can perform before it fails: a NO [LIMIT] Requested sort would have taken too long The SORT reply is still returned, but it's likely not correct.

lib-storage: Fix error handling when sorting mails. All errors were treated the same as if message had been expunged. This caused potentially wrong results to be sent to the client without any indication that they're wrong.

Reversing the primary sort criterion reversed also reversed secondary criterions. Fixed reverse sorting the first condition. --HG-- branch : HEAD

Message sort index handling rewrite to fix several race conditions when multiple processes are sorting at the same time. Also fixes sorting to work correctly with 64bit timestamps and file sizes. --HG-- branch : HEAD