Address an allocation TODO in the default ServerSelector.

And add a special case for only 1 server.

Minor win, in any case.

benchmark                      old ns/op     new ns/op     delta
BenchmarkPickServer            262           234           -10.69%
BenchmarkPickServer_Single     253           139           -45.06%

benchmark                      old allocs     new allocs     delta
BenchmarkPickServer            1              0              -100.00%
BenchmarkPickServer_Single     1              0              -100.00%

benchmark                      old bytes     new bytes     delta
BenchmarkPickServer            8             0             -100.00%
BenchmarkPickServer_Single     8             0             -100.00%