When a connection is hijacked, release the reference to the bufio.Writer
that is used with the chunkWriter.  The chunkWriter is not used after
the connection is hijacked.

Also add a test to check that double Hijack calls do something sensible.

benchmark                old ns/op    new ns/op    delta
BenchmarkServerHijack        24137        20629  -14.53%

benchmark               old allocs   new allocs    delta
BenchmarkServerHijack           21           19   -9.52%

benchmark                old bytes    new bytes    delta
BenchmarkServerHijack        11774         9667  -17.90%

R=bradfitz, dave, chris.cahoon
CC=golang-codereviews
https://golang.org/cl/39440044