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.

The results of benchmarks are:

$ ./node-slow bench && ./node-fast bench
4997.4 ns/op
4685.7 ns/op

NOTE: `fast` - was a patched node.js, `slow` - unpatched node.js with vanilla v8.

BUG=

Review URL: https://codereview.chromium.org/1316873004

Cr-Commit-Position: refs/heads/master@{#30495}