add a multi-codec test case for preferCodec

* re-enable setting for enabling/disabling rtx

* add tests for rtxmodifier.  (and a bugfix found by the tests)

* remove rtx handling from sdp-consistency (it will be handled separately)

* better error printing in JingleSessionPC around renegotiation

* add rtx modifier to handle inserting local rtx ssrcs, keeping local rtx ssrcs consistent, and imploding remote rtx ssrcs

* jsdoc, es6 cleanup, logging tweaks

* fix line that's too long

* add a 'parseGroupSsrcs' helper function

* fix a bug when passing group ssrcs to be parsed

* address pr feedback

* address pr feedback

* fix for loop

* address pr feedback

* better const in SampleSdpStrings

The function filter_special_chars does not have to throw a TypeError
when its argument text is null or undefined because (1) it can be argued
that neither of these two contains special characters and (2) it is much
more convenient to have the check in the function in question rather at
its multiple call sites.