Accept nil Candidate in AddRemoteCandidate

Allow a user to pass a nil Candidate. We perform no actions off of this
currently. Until browsers implement end-of-candidates consistently it
isn't something we can do.

Relates to pion/webrtc#1212 and pion#271

Update CI configs to v0.4.17

Update lint scripts and CI configs.

Update module pion/dtls/v2 to v2.0.4

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Use candidate context in validateNonSTUNTraffic

This operation needs to be cancelled when the candidate is closed.
Before it was only cancelled when the Agent was closed.

Resolves pion#299

Log don't return error on writeTo fail

Downstream isn't able to distinguish a temporary from a Permanent
error yet.

Relates to pion#252

ICE-Lite ControlledSelector now validates pairs

Fixes pion#288

Add proxy dialer interface

Enable user to implement the golang.org/x/net/proxy
dialer interface in order to have a customized dialer.
The customized dialer could be one that connect through
a corporate HTTP/HTTPS proxy.

Resolves pion#284

Update module pion/dtls/v2 to v2.0.3

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Expose Priority and Foundation overrides

Allow pion/webrtc to override these as well

Preserve Priority and Foundation E2E

We need these when handling remote candidates