Makes the wrapper classes take boxed query handles. Also cleans up some
repeated code patterns. It also encapsulates the QueryHelper class more
so that the pool indexes and handles are hidden from the user of the
class.

Fixed while working on GPU trace event timing.

Bug: angleproject:4433
Change-Id: Ib6cba9c52ec956ebede9b411b70261ea5b877d7d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2081378
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>