buffer: construct Uint8Array in JS

Overall construction time of Typed Arrays is faster in JS, but the
problem with using it normally is zero-fill of memory. Get around this
by using a flag in the ArrayBuffer::Allocator to trigger when memory
should or shouldn't be zero-filled.

Remove Buffer::Create() as it is no longer called.

The creation of the Uint8Array() was done at each callsite because at
the time of this patch there was a performance penalty for centralizing
the call in a single function.

PR-URL: https://github.com/nodejs/node/pull/2866


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