Enable add/remove routes after server starts

[yangchenyun]

• Use a map instead of slice to hold routes for each configuration
• Issue a serial number for the route added in order to remove them later
• Ensure Add/RemoveRoute is thread-safe by using a lock to guard write to the routes, because each write to router has its own serial number, racing condition only exists during registration. The lock should have minimal affect during serving.

Also added two test helpers to simplify and complete existing tests:

func testRouteToBackend(t *testing.T, front net.Listener, back net.Listener, msg string) 
func testNotRouteToBackend(t *testing.T, front net.Listener, back net.Listener, msg string) <-chan bool

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[yangchenyun]

I signed it!

[googlebot]

CLAs look good, thanks!

[bradfitz]

All the renamings in this PR obscure the actual change you're trying to make.

Can you split your changes into two parts, or refrain from renames for now?

[yangchenyun]

I separated the test refactoring in a #16, and I would do rebase and squash after that is check in.

[georgmu]

Adding routes only works for an already registered ipPort. The stuff in done in tcpproxy.Start() - looping over configs and calling p.netListen() is not done again.

For removal of a route, it would be nice to cancel onging connections for this route. This would require a map or list of ongoing connections for a route