Adds 2 new methods to v2 plugin `Acquire` and `Release` which allow
refcounting directly at the plugin level instead of just the store.
Since a graphdriver is initialized exactly once, and is really managed
by a separate object, it didn't really seem right to call
`getter.Get()` to refcount graphdriver plugins.
On shutdown it was particularly weird where we'd either need to keep a
driver reference in daemon, or keep a reference to the pluggin getter in
the layer store, and even then still store extra details on if the
graphdriver is a plugin or not.
Instead the plugin proxy itself will handle calling the neccessary
refcounting methods directly on the plugin object.
Also adds a new interface in `plugingetter` to account for these new
functions which are not going to be implemented by v1 plugins.
Changes terms `plugingetter.CREATE` and `plugingetter.REMOVE` to
`ACQUIRE` and `RELEASE` respectively, which seems to be better
adjectives for what we're doing.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
f29bbd16f5