src: micro-optimize ALPN negotiation

99 out of a 100 times (conservative estimate!) the negotiated protocol
will be either "h2" or "http/1.1" so reuse an existing JS string for
those instead of creating a new one every time.

PR-URL: https://github.com/nodejs/node/pull/26836


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>