Properly sanitize STUN urls

Currently we error if a STUN URL passed to Configuration.ICEServers
contained a query parameter. Fix this by using the getICEServers
function that sanitizes these for us.

Add SettingEngine entries needed for offline sync

Allow users to set statice ICECredentials and to disable
DTLS Certificate Verification

Check for dc.id in stats

Recent changes allow a dc to have a nil id before it's actually open,
but stats used to require the id.

Allow user to Close DataChannel before signaling

Match W3C WebRTC and allow a DataChannel to close even before
signaling happens. When SCTPTransport opens make sure
to check the readyState first.

Update module pion/dtls to v2.0.0-rc.2

Bumping dtls to support backwards compatibility
to Go 1.12

Use pion/dtls v2

API now allows multiple certificates, there is no behavior change
this is just to bring things more inline with crypto/tls

Update module pion/dtls to v1.5.3

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Fix all routine leaks in tests

Fix tests that didn't properly close and add
test.CheckRoutines everywhere. No changes in pion/webrtc
but we did catch hanging thread in pion/ice

Expose Answering DTLSRole via SettingEngine

User can now control what DTLSRole the local client
takes when answering via the SettingEngine.

Relates to pion#880

Update module pion/datachannel to v1.4.11

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