Pin RethinkDB to a concrete v8 version and fetch it during build

[coffeemug]

Talked to @AtnNn about it, we agreed to do what other software with this problem does -- pin to a v8 version and update it at our convenience.

[anatol]

This is fine with me (as a package maintainer). I assume you'll be bumping v8 version periodically.

FYI current stable version is 3.22 according to http://omahaproxy.appspot.com/

[AtnNn]

I updated #1510 with some #defines to be able to build against older versions of v8 and I've bumped the v8 version to 3.22.24.17.

As part of its build process, v8 downloads and builds icu.

I have also tested building rethinkdb with the dev version of v8 (3.24.30.1) and there are lots of errors, the API seems to have changed a lot.

[AtnNn]

For #1231 I am avoiding fetching anything during the build process. To close this issue, I will include v8 in the source distribution (http://download.rethinkdb.com/dist).