Splunk: Servers failover

upgrade versions

Conflicts:
	spring-integration-splunk/gradle.properties

upgrade gradle version

revert spring integration upgrade version

Add a feature to be able to declare more than one splunk server.
It's a simple failover mechanism with any pooling.
Before give back the Service instance, the framework just try a getInfo call.
To preserve backward compat this check it's not activated per default but only
if the splunk server instance is marked with testOnBorrow.

Sample:
````xml
  <int-splunk:server id="splunkServer" host="${splunk.server.host}" port="${splunk.server.port}"
                     username="${splunk.server.username}" password="${splunk.server.password}"
                     owner="${splunk.server.owner}" scheme="${splunk.server.scheme}" testOnBorrow="true" />

  <int-splunk:server id="splunkServerBackup" host="localhost" port="9999"
                     username="${splunk.server.username}" password="${splunk.server.password}"
                     owner="${splunk.server.owner}" scheme="${splunk.server.scheme}" testOnBorrow="true" />

  <util:list id="splunkServersList">
    <ref bean="splunkServer" />
    <ref bean="splunkServerBackup" />
  </util:list>

  <bean id="splunkServiceFactory" class="org.springframework.integration.splunk.support.SplunkServiceFactory">
    <constructor-arg ref="splunkServersList"/>
  </bean>

  <int-splunk:inbound-channel-adapter id="splunk-notify-order-status-change-channel"
                                      auto-startup="true"
                                      search=""
                                      splunk-server-ref="splunkServer;splunkServerBackup"
                                      channel="notify-order-status-change-input"
                                      mode="BLOCKING"
                                      init-earliest-time="${splunk.order-status-change.init-earliest-time}"
                                      >
    <int:poller fixed-rate="${splunk.order-status-change.pooling.rate.time}" time-unit="SECONDS"/>
  </int-splunk:inbound-channel-adapter>
````

some changes due to pr comments

Conflicts:
	spring-integration-splunk/gradle.properties

formatting: tabs instead of spaces.....

fix documentation with failover mechanism

formatting

formatting

upgrade versions

some changes due to pr comments

fix javadoc issues

1.1 xsd file

Polishing and upgrading