RPC endpoints for the user-driven APIs (`UpsertOneTimeToken` and
`ExchangeOneTimeToken`) and token expiration (`ExpireOneTimeTokens`).
Includes adding expiration to the periodic core GC job.
If ACL Request is unauthenticated, we should honor the anonymous token.
This PR makes few changes:
* `GetPolicy` endpoints may return policy if anonymous policy allows it,
or return permission denied otherwise.
* `ListPolicies` returns an empty policy list, or one with anonymous
policy if one exists.
Without this PR, the we return an incomprehensible error.
Before:
```
$ curl http://localhost:4646/v1/acl/policy/doesntexist; echo
acl token lookup failed: index error: UUID must be 36 characters
$ curl http://localhost:4646/v1/acl/policies; echo
acl token lookup failed: index error: UUID must be 36 characters
```
After:
```
$ curl http://localhost:4646/v1/acl/policy/doesntexist; echo
Permission denied
$ curl http://localhost:4646/v1/acl/policies; echo
[]
```
Noticed that ACL endpoints return 500 status code for user errors. This
is confusing and can lead to false monitoring alerts.
Here, I introduce a concept of RPCCoded errors to be returned by RPC
that signal a code in addition to error message. Codes for now match
HTTP codes to ease reasoning.
```
$ nomad acl bootstrap
Error bootstrapping: Unexpected response code: 500 (ACL bootstrap already done (reset index: 9))
$ nomad acl bootstrap
Error bootstrapping: Unexpected response code: 400 (ACL bootstrap already done (reset index: 9))
```
The ForceLeaveRequest endpoint may only be called on servers, but the
code was using a Client to resolve tokens. This would cause a panic when
an agent wasn't both a Server and a Client.
