Allow setting limits on entities returned

[ramnivas]

Currently, when executing a query such as the following, Exograph returns all products.

products {
  id
  name
}

We should:

1. Allow configuring a global, say, EXO_QUERY_ENTRIES_LIMIT (default to, say, 100)
2. Allow a per-type configuration (@maxLimit(100))

Then we automatically compute the effective limit as:

min(the `limit` parameter specified in the query, the configured limit)

where the configured limit is:

per-type configuration, if specified, or `EXO_QUERY_ENTRIES_LIMIT`

Then we execute the query as if the user has specified the limit parameter:

products(limit: <the computed limit>) {
  id
  name
}

Open question:
We need to indicate to the caller that their limit (either explicitly specified or implied as Infinity) was overridden. What's the best way to do it?