Graylog server won't start with default setting "elasticsearch_idle_timeout = -1s"

## Expected Behavior

Server should start with default settings.

## Current Behavior

Server does not start, when enabling the config setting **elasticsearch_idle_timeout** with it's default value "-1s":

```
# Maximum idle time for an Elasticsearch connection. If this is exceeded, this connection will
# be tore down.
#
# Default: inf
#elasticsearch_idle_timeout = -1s
```

The server does not start up, the log says:

```
2019-04-12T15:19:54.103+02:00 ERROR [CmdLineTool] Invalid configuration
com.github.joschi.jadconfig.ParameterException: Couldn't convert value for parameter "elasticsearch_idle_timeout"
at com.github.joschi.jadconfig.JadConfig.processClassFields(JadConfig.java:141) ~[graylog.jar:?]
at com.github.joschi.jadconfig.JadConfig.process(JadConfig.java:99) ~[graylog.jar:?]
at org.graylog2.bootstrap.CmdLineTool.processConfiguration(CmdLineTool.java:351) [graylog.jar:?]
at org.graylog2.bootstrap.CmdLineTool.readConfiguration(CmdLineTool.java:344) [graylog.jar:?]
at org.graylog2.bootstrap.CmdLineTool.run(CmdLineTool.java:178) [graylog.jar:?]
at org.graylog2.bootstrap.Main.main(Main.java:50) [graylog.jar:?]
Caused by: com.github.joschi.jadconfig.ParameterException: Couldn't convert value "-1s" to Duration.
at com.github.joschi.jadconfig.converters.DurationConverter.convertFrom(DurationConverter.java:25) ~[graylog.jar:?]
at com.github.joschi.jadconfig.converters.DurationConverter.convertFrom(DurationConverter.java:12) ~[graylog.jar:?]
at com.github.joschi.jadconfig.JadConfig.convertStringValue(JadConfig.java:167) ~[graylog.jar:?]
at com.github.joschi.jadconfig.JadConfig.processClassFields(JadConfig.java:139) ~[graylog.jar:?]
... 5 more
Caused by: java.lang.IllegalArgumentException: Invalid duration: -1s
at com.github.joschi.jadconfig.util.Duration.parse(Duration.java:77) ~[graylog.jar:?]
at com.github.joschi.jadconfig.converters.DurationConverter.convertFrom(DurationConverter.java:23) ~[graylog.jar:?]
at com.github.joschi.jadconfig.converters.DurationConverter.convertFrom(DurationConverter.java:12) ~[graylog.jar:?]
at com.github.joschi.jadconfig.JadConfig.convertStringValue(JadConfig.java:167) ~[graylog.jar:?]
at com.github.joschi.jadconfig.JadConfig.processClassFields(JadConfig.java:139) ~[graylog.jar:?]
... 5 more
```

Obviously, "-1s" is not "inf".

## Possible Solution

Setting a reasonable default value for **elasticsearch_idle_timeout**.

## Steps to Reproduce (for bugs)
1. Remove comment sign here:
```
#elasticsearch_idle_timeout = -1s
```
(https://github.com/Graylog2/graylog2-server/blob/master/misc/graylog.conf#L202)

2. Restart graylog-server
3. Watch the logs

## Your Environment

* Graylog Version: 3.0.1-2
* Elasticsearch Version: 6.6.0