Revert "Remove old copy of server.go and use the library"

This reverts commit 4b179726f973891061b79ea76ac17bd9de0d04ec.

[#62818290] The commit we are reverting removed modifications to our
custom copy of net/http/server.go from the Go 1.0.3 standard library,
including changes in 1a8c32646473bbba29e00e08690444a649d80f42 that were
apparently designed to "[m]ake response independent of [the] request". We are
changing back to the modified version in an attempt to prevent frequent
gorouter crashes.

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>