incoming conn race condition

[hhalpin]

From @david415 on October 7, 2018 2:46

================== WARNING: DATA RACE Write at 0x00c4200c00f0 by goroutine 57: github.com/katzenpost/core/wire.(Session).RecvCommand() /home/user/gopath/src/github.com/katzenpost/core/wire/session.go:371 +0xb2 github.com/katzenpost/server/internal/incoming.(incomingConn).worker.func2() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/incoming_conn.go:203 +0x98

Previous read at 0x00c4200c00f0 by goroutine 64: github.com/katzenpost/core/wire.(Session).PeerCredentials() /home/user/gopath/src/github.com/katzenpost/core/wire/session.go:431 +0x40 github.com/katzenpost/server/internal/incoming.(listener).IsConnUnique() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/listener.go:147 +0x24a github.com/katzenpost/server/internal/incoming.(*incomingConn).worker() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/incoming_conn.go:185 +0x944

Goroutine 57 (running) created at: github.com/katzenpost/server/internal/incoming.(*incomingConn).worker() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/incoming_conn.go:200 +0xb27

Goroutine 64 (running) created at: github.com/katzenpost/server/internal/incoming.(listener).onNewConn.func1() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/listener.go:109 +0x5c github.com/katzenpost/server/internal/incoming.(listener).onNewConn() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/listener.go:112 +0x12f github.com/katzenpost/server/internal/incoming.(listener).worker() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/listener.go:96 +0x321 github.com/katzenpost/server/internal/incoming.(listener).(github.com/katzenpost/server/internal/incoming.worker)-fm() /home/user/gopath/src/github.com/katzenpost/server/internal/incoming/listener.go:178 +0x41 github.com/katzenpost/core/worker.(*Worker).Go.func1() /home/user/gopath/src/github.com/katzenpost/core/worker/worker.go:38 +0x5c

Copied from original issue: katzenpost/server#84