CamelIMAPXStoreSummary cleanups.
CamelIMAPXStoreSummary: Don't bother with store_info_set_string().

Just set the CamelIMAPXStoreInfo member directly.
CamelIMAPXStoreSummary: Add separator characters to the file format.

This bumps the store summary file format version to store hierarchy
separator characters alongside server mailbox names.  The separator
character can help identify a mailbox's namespace more reliably.