EphemeralSocket orphaned setInterval causes lingering timeout

I noticed issues running sdc in a CLI app, when I called `sdc.close()` the process seemed to hang.

I ran it with timers-debug (https://github.com/tenthbitinc/timers-debug) and noticed that the EphemeralSocket had a orphaned setInterval. It looks like it was just an issue with _createSocket() not checking if there already was an existing timeout set.

```
$ node examples/close-test.js
setTimeout 1
timeoutFired 1
send( statsd-client.test.gauge:77.02251526061445|g )
_enqueue( statsd-client.test.gauge:77.02251526061445|g )
setInterval 2
send( statsd-client.test.counter:1|c )
_enqueue( statsd-client.test.counter:1|c )
send( statsd-client.test.counter:1|c )
_enqueue( statsd-client.test.counter:1|c )
send( statsd-client.speed:59|ms )
_enqueue( statsd-client.speed:59|ms )
close()
setTimeout 3
_socketTimeout()
_flushBuffer() → statsd-client.test.gauge:77.02251526061445|g
statsd-client.test.counter:1|c
statsd-client.test.counter:1|c
statsd-client.speed:59|ms
_send( statsd-client.test.gauge:77.02251526061445|g
statsd-client.test.counter:1|c
statsd-client.test.counter:1|c
statsd-client.speed:59|ms )
_createSocket()
setInterval 4
_createSocket setInterval { ontimeout: [Function], debugUniqueID: 4 }
statsd-client.test.gauge:77.02251526061445|g
statsd-client.test.counter:1|c
statsd-client.test.counter:1|c
statsd-client.speed:59|ms
_socketTimeout()
_socketTimeout()
close()
clearInterval 4
setTimeout 5
timeoutFired 5
kill()
_socketTimeout()
_socketTimeout()
_socketTimeout()
_socketTimeout()
timeoutFired 3
active timer #Error
    at TimersDebug.uniqueIDForTimer (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:95:19)
    at TimersDebug.setInterval (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:73:25)
    at EphemeralSocket._enqueue (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/EphemeralSocket.js:136:29)
    at EphemeralSocket.send (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/EphemeralSocket.js:163:14)
    at StatsDClient.gauge (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/statsd-client.js:41:27)
    at /Users/ddaniels/Dropbox/code/github/node-statsd-client/examples/close-test.js:15:13
    at null._onTimeout (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:57:9)
    at Timer.listOnTimeout [as ontimeout] (timers.js:110:15) Error
    at TimersDebug.uniqueIDForTimer (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:95:19)
    at TimersDebug.setInterval (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:73:25)
    at EphemeralSocket._enqueue (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/EphemeralSocket.js:136:29)
    at EphemeralSocket.send (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/EphemeralSocket.js:163:14)
    at StatsDClient.gauge (/Users/ddaniels/Dropbox/code/github/node-statsd-client/lib/statsd-client.js:41:27)
    at /Users/ddaniels/Dropbox/code/github/node-statsd-client/examples/close-test.js:15:13
    at null._onTimeout (/Users/ddaniels/Dropbox/code/github/node-statsd-client/node_modules/timers-debug/timers-debug.js:57:9)
    at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
_socketTimeout()
_socketTimeout()
```