* implement a memory-optimized time.Time replacement
* monotime: properly handle systems with bad timer resolution (Windows)
* monotime: simplify Since
* store paths in a slice, not a map
No functional change expected.
* save the timestamp of the last received packet for a path
* evict old paths
* reduce the path timeout to 5s
* make it possible to pack path probes with multiple frames
* simplify function signature of pathManager.HandlePacket
* simplify connection short header packet handling logic
No functional change expected.
* make server send PATH_RESPONSEs on the same path
This makes sure that we’re actually testing for return routability.
