mesh: distance too large for preamble detect, and deterministic collisions

Update: actually there are 2 issues:

1. The nodes in the mesh example are too far away when preamble detection is active, and
2. The intermediate nodes forward packets immediately, leading to a 100% probability of collisions.

Point 1 can be fixed "easily", point 2 requires further work.

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Reported by Eduardo Soares on Zulip.

src/mesh/examples/mesh.cc is broken - meaning that nodes do not communicate with each other. The PCAP files only show each node's own beacons.

The culprit is in the wifi preamble detection. Adding the line:

  wifiPhy.DisablePreambleDetectionModel ();

at line 168 of said example restores the previous behaviour.

Another option is to lower the default step value (I tested with 50, default is 100), because making the nodes closer to each other allows them to actually exchange packets.