As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.
Tests expect that as soon as eventer shuts down immediately on context
cancellations; but golang does not guarantee priority when multiple
pending channels are ready in a select statement.
This PR plumbs the plugins done ctx through the base and driver plugin
clients (device already had it). Further, it adds generic handling of
gRPC stream errors.
