heap: make array buffer maps disjoint

heap: make array buffer maps disjoint

Remove intersection from the `std::map`s representing current live
ArrayBuffers. While being simpler to understand, it poses significant
performance issue for the active ArrayBuffer users (like node.js).

Store buffers separately, and process them together during mark-sweep
phase.

BUG=
R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1326613002

Cr-Commit-Position: refs/heads/master@{#30539}