client: various race fixes

[rjeczalik]

Fixes all races, old and new ones. All, since running -race with stress goes ok without failures until all available socket fds get depleted ;)

$ go test -race -c .
$ stress ./tunnel.test                                                                                                                                                                                                                            
0 runs so far, 0 failures
8 runs so far, 0 failures
9 runs so far, 0 failures
16 runs so far, 0 failures
17 runs so far, 0 failures
20 runs so far, 0 failures
...

/cc @cihangir

[cihangir]

:100: