lib-storage: Avoid index refresh with mailbox_list_index_very_dirty_syncs=yes

global: unsigned int:1 -> bool:1 perl -i -pe 's/unsigned int ([^,:;]+):1;/bool $1:1;/' **/*.[ch]

lib-storage: Partially reverted previous change. Added mailbox_list_index_sync_delete() instead.

lib-storage: Added mailbox_list_index_delete_entry()

lib-storage: Added initial implementation for layout=index. The idea is that all of the mailbox names only exist in the mailbox list index. Storage directories use mailbox GUIDs. This avoids all kinds of race conditions with mailbox renames. mailbox_list_index=yes is required for this layout to work. There are probably still some race conditions within the indexing code itself. Also error recovery is missing.