Default candidate pool size

https://github.com/rtcweb-wg/jsep/issues/12

JSEP says:
S 4.1.1.
" The PeerConnection constructor allows the application to specify
global parameters for the media session, such as the STUN/TURN servers
and credentials to use when gathering candidates. The size of the ICE
candidate pool can also be set, if desired; by default the candidate
pool size is zero."

The WebRTC spec says:
"At this point the ICE Agent does not know how many ICE components it
needs (and hence the number of candidates to gather), but it can make
a reasonable assumption such as 2. As the RTCPeerConnection object
gets more information, the ICE Agent can adjust the number of
components"

These need to be harmonized.

My personal opinion is that candidate pooling is useful here and we
should probably leave the default in the hands of the browser. I
could live with 0 however.

-Ekr