We had a funny bug where the Handle Allocator would re-allocate
reserved handles after the app layer creates one with Bind rather
than using Gen. This affects Textures, Buffers and Renderbuffers.

Fix this by using a different allocation scheme. It should still
be fast on the "good" case (using Gen) and use tree lookups on the
bind case. Also add some unit tests.

BUG=angleproject:942

Change-Id: I2e3f4c31b28cb86bd3699035f5d55568a2a1d7d5
Reviewed-on: https://chromium-review.googlesource.com/258904Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>