new_jsep_session_description: session-version doesn't follow the spec

[vsotog]

Current implementation:

  session_version: SystemTime::now()
                .duration_since(UNIX_EPOCH)
                .unwrap_or_else(|_| Duration::from_secs(0))
                .subsec_nanos() as u64

Spec:
https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.2.2

5.2.2. Subsequent Offers

   o  The fields of the "o=" line MUST stay the same except for the
      <session-version> field, which MUST increment by one on each call
      to createOffer if the offer might differ from the output of the
      previous call to createOffer; implementations MAY opt to increment
      <session-version> on every call.  The value of the generated
      <session-version> is independent of the <session-version> of the
      current local description; in particular, in the case where the
      current version is N, an offer is created and applied with version
      N+1, and then that offer is rolled back so that the current
      version is again N, the next generated offer will still have
      version N+2.

MUST increment by one on each call to createOffer

https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.3.2

5.3.2. Subsequent Answers

   o  The fields of the "o=" line MUST stay the same except for the
      <session-version> field, which MUST increment if the session
      description changes in any way from the previously generated
      answer.

Related to: https://github.com/pion/sdp/issues/60