lib-storage: Add mail_vsize_bg_after_count setting.

lib-storage: Add mail_vsize_bg_after_count setting.

If folder vsize calculation requires opening more than this many mails from
disk (i.e. mail sizes aren't in cache already), return failure and finish
the calculation via indexer process.

This should be used like:

protocol !indexer-worker {
  mail_vsize_bg_after_count = 10
}