v8: upgrade to v8 3.20.7
src: update v8 build requirements

In v8 3.20.4 build/common.gypi was moved to build/toolchain.gypi and
build/features.gypi.
v8: reapply floating patches
util: removed duplicated isArray check
stream: Simplify flowing, passive data listening

Closes #5860

In streams2, there is an "old mode" for compatibility.  Once switched
into this mode, there is no going back.

With this change, there is a "flowing mode" and a "paused mode".  If you
add a data listener, then this will start the flow of data.  However,
hitting the `pause()` method will switch *back* into a non-flowing mode,
where the `read()` method will pull data out.

Every time `read()` returns a data chunk, it also emits a `data` event.
In this way, a passive data listener can be added, and the stream passed
off to some other reader, for use with progress bars and the like.

There is no API change beyond this added flexibility.