Allow connecting to event stream at current index

### Nomad version
1.0.0-beta3

### Issue
The event stream allows specifying an `index` query parameter, indicating the [desired start](https://www.nomadproject.io/api-docs/events#index). The default value is `0`, which plays back all available events.

It would be nice to have the ability to start at the current index. This can be done now by making an API call (e.e., `/v1/namespaces`) and checking the `X-Nomad-Index` response header, then using that on the subsequent call to the event stream. Supporting this use case in the `/v1/event/stream` endpoint (for example, using `?index=-1`) would avoid making two calls.

Start at the /v1/event/stream endpoint and trace its index parameter handling and existing event-stream tests. Verify how the current index is obtained and define the sentinel behavior, then add coverage showing that a connection starts at the current index without a separate API request.

Written by the indexing model from the issue text.