lib-storage: INDEX_LIST_STORAGE_CONTEXT now requires context Satisfies static analyzers

LAYOUT=index: Fix updating STATUS changes in mailbox list index Mailbox list index backend code was overriding sync_init and sync_deinit methods, which STATUS handling had already already overridden. They both used the same super struct, so STATUS's sync_* were never called.

lib-storage: Try to avoid opening mailbox list index on mailbox access Perform the STATUS (and other relevant) updates only if syncing or transaction commits changed anything. This could be optimized further to check even more strongly whether the seen changes could cause changes to the list index, but it's probably not worth the effort.

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.