Extmap: Add ssrc-audio-level RTP extension URI

This is for ion-sfu to be able to register this RTP extension. From
RFC6464.

Also add myself to the README as required by Metadata.Contributors CI
check.

Improve Unmarshal performance by 11%

* strings.Fields() and strconv.ParseInt() replaced by
  new functions: lexer.readField() and lexer.readUint64Field()

* Because sdp size is small, lexer now uses just simple []byte

* New parseError show invalid position

* Bug in tests messages: bytes wasn't converted to strings before output

* Bug in ConnectionInformation.String(): nil pointer dereference

Benchmarks (before/after)
BenchmarkUnmarshal-12 156142 6981 ns/op 4008 B/op 108 allocs/op
BenchmarkUnmarshal-12 167923 6193 ns/op 2584 B/op 108 allocs/op

Update CI configs to v0.4.7

Update lint scripts and CI configs.

Update CI configs to v0.4.6

Update lint scripts and CI configs.

Remove ICE logic and place into pion/ice

Remove pion/ice dependencies. This isn't SDP specific, and complicates
the dep tree for users who just want ICE

Fix session ID generation

Use crypto random according to
https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.2.1

Test for MediaDescription.WithFingerprint

This tests that an attribute is added, and the key
and value of that attribute are as expected.

Add extmap consts

Add ext consts and update readme

Add media flow attributes

sendrecv - Used to establish a 2-way media stream.
recvonly - The SIP endpoint would only receive (listen mode) and not
           send media.
sendonly - The SIP endpoint would only send and not receive media.
inactive -  The SIP endpoint would neither send nor receive media.

Add AttrKeyMsid

Related to pion/webrtc#1135