API queries can request pagination using the `NextToken` and `PerPage`
fields of `QueryOptions`, when supported by the underlying API.

Add a `NextToken` field to the `structs.QueryMeta` so that we have a
common field across RPCs to tell the caller where to resume paging
from on their next API call. Include this field on the `api.QueryMeta`
as well so that it's available for future versions of List HTTP APIs
that wrap the response with `QueryMeta` rather than returning a simple
list of structs. In the meantime callers can get the `X-Nomad-NextToken`.

Add pagination to the `Eval.List` RPC by checking for pagination token
and page size in `QueryOptions`. This will allow resuming from the
last ID seen so long as the query parameters and the state store
itself are unchanged between requests.

Add filtering by job ID or evaluation status over the results we get
out of the state store.

Parse the query parameters of the `Eval.List` API into the arguments
expected for filtering in the RPC call.

Refactor to expose only a Page method that populates the result set by
repeatedly calling an append function over the paged iterator.