Reuse of closed m= sections in Firefox

[snnz]

Due to solution to issue #104, SDP grows indefinitely in Firefox if multiple producers are created. Firefox implements closed m= sections reuse, but instead of first available closed sections, it chooses only closed sections of the kind matching the kind of track of the new transceiver. If a section of the same kind is not found, it creates a new one, even if there are other closed sections. Because of this, the method that predicts the index of the new section doesn't work with Firefox. However, this can be worked around by searching local offer SDP for the new transceiver's mid and rearranging remote SDP sections according to the order of sections in the local offer. See PR #149 for the example of implementation.