Having this type live in the agent/consul package makes it difficult to
put anything that relies on token resolution (e.g. the new gRPC services)
in separate packages without introducing import cycles.
For example, if package foo imports agent/consul for the ACLResolveResult
type it means that agent/consul cannot import foo to register its service.
We've previously worked around this by wrapping the ACLResolver to
"downgrade" its return type to an acl.Authorizer - aside from the
added complexity, this also loses the resolved identity information.
In the future, we may want to move the whole ACLResolver into the
acl/resolver package. For now, putting the result type there at least,
fixes the immediate import cycle issues.
As part of removing the legacy ACL system ACL upgrading and the flag for
legacy ACLs is removed from Clients.
This commit also removes the 'acls' serf tag from client nodes. The tag is only ever read
from server nodes.
This commit also introduces a constant for the acl serf tag, to make it easier to track where
it is used.
