* Simplify Run(): the function that was being sent over a channel doesn't
need to close over anything except latestToken, and we don't need to
create a new one each iteration.  Instead just pass the relevant items,
namely the token and sink to work on.

* Disallow the following config combinations:
1. auto_auth.method.wrap_ttl > 0 and multiple file sinks
2. auto_auth.method.wrap_ttl > 0 and single file sink with wrap_ttl > 0
3. auto_auth.method.wrap_ttl > 0 and cache.use_auto_auth_token = true

* Expose errors that occur when APIProxy is forwarding request to Vault.

* Fix merge issues.