src: drop homegrown thread pool, use libplatform

Drop the homegrown thread pool that was introduced in commit 50839a04
("v8_platform: provide default v8::Platform impl") and use one from
V8's libplatform library.  Performance is comparable and it removes
a few hundred lines of code.

The calls to v8::platform::PumpMessageLoop() are currently no-ops
because V8 does not (yet?) use v8::Platform::CallOnForegroundThread().

Packagers that link against a shared libv8 now also need to make
libv8_platform available.

PR-URL: https://github.com/iojs/io.js/pull/1329


Reviewed-By: Fedor Indutny <fedor@indutny.com>