* Fix CCA detection in Cooja in the case when the receiver swicthes on the right channel during an ongoing transmission. Always add a connection on transmission, even when the channel is not correct. Initially the connection is in a dormant state; this mimics what Cooja is doing when the receiver radio is turned off;
when the receiver is turned on and switched to the right channel, `updateSignalStrengths()` is called, and the connection starts to recieve PHY-level traffic.
* Add "channel" property for DGRM edges.
* Avoid cross-channel interference on DGRM and MRM radio mediums
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