Use sync.Pool for logger Messages

This reduces allocs and bytes used per log entry significantly as well
as some improvement to time per log operation.

Each log driver, however, must put messages back in the pool once they
are finished with the message.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/logger/splunk/splunk.go

@@ -336,7 +336,7 @@ func (l *splunkLoggerInline) Log(msg *logger.Message) error {
 	event.Source = msg.Source
 
 	message.Event = &event
-
+	logger.PutMessage(msg)
 	return l.queueMessageAsync(message)
 }
 
@@ -354,7 +354,7 @@ func (l *splunkLoggerJSON) Log(msg *logger.Message) error {
 	event.Source = msg.Source
 
 	message.Event = &event
-
+	logger.PutMessage(msg)
 	return l.queueMessageAsync(message)
 }
 
@@ -362,7 +362,7 @@ func (l *splunkLoggerRaw) Log(msg *logger.Message) error {
 	message := l.createSplunkMessage(msg)
 
 	message.Event = string(append(l.prefix, msg.Line...))
-
+	logger.PutMessage(msg)
 	return l.queueMessageAsync(message)
 }