The idea is that the user's question is "is this folder's data available
offline?" and not "does this folder have AlwaysLocal pin state?".
The the answers to the two questions can differ: an always-local
folder can have subitems that are not always-local and are dehydrated.
The new availability enum intends to describe the answer to the user's
actual question and can be derived from pin states. If pin states aren't
stored in the database the way of calculating availability will depend
on the vfs plugin.
That just complicated things. It's ok if Vfs is not a fully abstract
interface class.
The pinstate-in-db methods are instead provided directly on Vfs and
VfsSuffix and VfsOff use them to implement pin states.
The start() method is simply non-virtual and calls into startImpl() for
the plugin-specific startup code.
- SyncJournalDB functions now behind internalPinStates() to avoid
accidental usage, when nearly everyone should go through Vfs.
- Rename Vfs::getPinState() to Vfs::pinState()
