* agent: Changed the `consul exec` ACL resolution logic to use the `acl_agent_token` if it's available. This lets operators configure an `acl_agent_token` with the required `write` privilieges to the `_rexec` prefix of the KV store without giving this to the `acl_token`, which would expose those privileges to users as well. [GH-3160]
* api: Added the ability to pass in a `context` as part of the `QueryOptions` during a request. This provides a way to cancel outstanding blocking queries. [GH-3195]
* api: Changed signature for "done" channels on `agent.Monitor()` and `session.RenewPeriodic` methods to make them more compatible with `context`. [GH-3271]
* docs: Added a complete end-to-end example of ACL bootstrapping in the [ACL Guide]https://www.consul.io/docs/guides/acl.html#bootstrapping-acls). [GH-3248]
* docs: Added a complete end-to-end example of ACL bootstrapping in the [ACL Guide](https://www.consul.io/docs/guides/acl.html#bootstrapping-acls). [GH-3248]
* vendor: Updated golang.org/x/sys/unix to support IBM s390 platforms. [GH-3240]