add missing eventHandler to StatsDTCPListener

[seruman]

When sending data with TCP exporter was panicking.

echo "foo_bar:1|c" | nc localhost 9125

INFO[0000] Starting StatsD -> Prometheus Exporter (version=, branch=, revision=)  source="main.go:168"
INFO[0000] Build context (go=go1.12.4, user=, date=)     source="main.go:169"
INFO[0000] Accepting StatsD Traffic: UDP :9125, TCP :9125, Unixgram   source="main.go:170"
INFO[0000] Accepting Prometheus Requests on :9102        source="main.go:171"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x1491d98]

goroutine 14 [running]:
main.(*StatsDTCPListener).handleConn(0xc000146ac0, 0xc000010048)
	/Users/selman/src/statsd_exporter/exporter.go:514 +0x208
created by main.(*StatsDTCPListener).Listen
	/Users/selman/src/statsd_exporter/exporter.go:489 +0x4a

In tests eventHandler was set for both listeners;

statsd_exporter/bridge_test.go

Line 296 in a276bac

l.SetEventHandler(&unbufferedEventHandler{c: events})

but not in main.go;

statsd_exporter/main.go

Line 205 in a276bac

tl := &StatsDTCPListener{conn: tconn}

[matthiasr]

Good catch, thank you!