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.
The pin state is a per-item attribute that has an effect on _type:
AlwaysLocal dehydrated files will be marked for hydration and OnlineOnly
hydrated files will be marked for dehydration.
Where exactly this effect materializes depends on how the pin states are
stored. If they're stored in the db (suffix) the dbEntry._type is
changed during the discovery.
If the pin state is stored in the filesystem, the localEntry._type must
be adjusted by the plugin's stat callback.
This patch makes pin states behave more consistently between plugins.
Previously with suffix-vfs pin states only had an effect on new remote
files. Now the effect of pinning or unpinning files or directories is as
documented and similar to other plugins.
