Thread safety

[esnible]

Small fixes to make the code safer.

I found these visually. They are not detected by any existing tests, and I ran the unit tests with -count 1000 -parallel 100.

We should probably write some tests for concurrent modifications, so we can catch these problems if more are introduced.

[maleck13]

I think it would also make sense to run the broker-router with the -race flag

go test -race mypkg   
$ go run -race mysrc.go 
$ go build -race mycmd