add some benchmarking for msg broadcasting via hub
only format SSEMessage once

I was concerned this would have more overhead for low numbers of
connections because of variable creation and garbage collection but
apparently that’s very cheap in Go.

Before
BenchmarkBroadcast1	 1000000	      2337 ns/op
BenchmarkBroadcast10	  200000	     12559 ns/op
BenchmarkBroadcast100	   10000	    120157 ns/op
BenchmarkBroadcast500	    5000	    637490 ns/op
BenchmarkBroadcast1000	    2000	   1259922 ns/op

After
BenchmarkBroadcast1	 1000000	      2304 ns/op
BenchmarkBroadcast10	  500000	      3955 ns/op
BenchmarkBroadcast100	  100000	     23111 ns/op
BenchmarkBroadcast500	   10000	    122018 ns/op
BenchmarkBroadcast1000	    5000	    269084 ns/op