retry on failed connections (at least some of them)

[sudermanjr]

If there are no servers currently, or if a connection fails, maybe retry a couple times? For no available servers, implement an incremental backoff or a consistent retry

└─ agones-allocator-client udp-demo
0 - got allocation 35.184.166.202 7851. Proceeding to connection...
I0622 19:37:33.665168   19372 udp.go:53] 0 - connected to gameserver and sending hello
1 - got allocation 35.184.166.202 7853. Proceeding to connection...
I0622 19:37:36.699382   19372 udp.go:53] 1 - connected to gameserver and sending hello
2 - got allocation 35.184.166.202 7352. Proceeding to connection...
I0622 19:37:39.680216   19372 udp.go:53] 2 - connected to gameserver and sending hello
3 - got allocation 35.184.166.202 7828. Proceeding to connection...
I0622 19:37:42.669916   19372 udp.go:53] 3 - connected to gameserver and sending hello
I0622 19:37:43.667673   19372 udp.go:61] 0 - sending EXIT command
E0622 19:37:46.097266   19372 udp.go:29] rpc error: code = ResourceExhausted desc = there is no available GameServer to allocate
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x14222af]

goroutine 43 [running]:
github.com/fairwindsops/agones-allocator-client/pkg/allocator.(*Client).testUDP(0xc00018a380, 0x4, 0xc000137c30)
        /Users/asuderma/go/src/github.com/fairwindsops/agones-allocator-client/pkg/allocator/udp.go:31 +0x10f
created by github.com/fairwindsops/agones-allocator-client/pkg/allocator.(*Client).RunUDPDemo
        /Users/asuderma/go/src/github.com/fairwindsops/agones-allocator-client/pkg/allocator/udp.go:18 +0x8c