item_get: bring back support for using `Find`

If there are query predicates apart from `id`, use `resource.Find` instead of `resource.Get`.
This puts back ability add additional predicates apart from `id`.
Who needs that, anyway?

Add a more complex schema benchmark (rs#78)

- Complex schema 1 is based off a request and Complex schema 2 is based off
 a response schema in real world schema usage.
- Move allocation of byte.Buffer outside of benchmark loop, as we're
 not interested in measuring that.
- Set Buffer to 512Kb to avoid dynamic schema growth for expected test
 usage.