Rollback pion/rtp to v0

Resolves pion#1908

Upgrade pion/rtp to v2

Also updates interceptor and srtp

Update module github.com/pion/ice/v2 to v2.1.10

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Avoid crash after a PC callback has been reset

We used to crash if a PC callback was reset, due to confusion
between a nil interface and an interface whose value is nil.

Fixes pion#1871

Update golang.org/x/net commit hash to 04defd4

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Refactored samplebuilder logic

Many corner cases would cause samplebuilder to fail and return invalid
results. This refactoring is more reliable in all cases.

Fixed bug in H264 writer by reusing the packet object in  H264 writer.

Expose SRTP protection profiles in setting engine

Allows to override default SRTP protection profiles in settingengine.go

Update module github.com/pion/ice/v2 to v2.1.7

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Update module github.com/pion/ice/v2 to v2.1.6

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Revert "Fixes MediaEngine: codec apt negotiation"

This reverts commit eeb67e1.