Add timer metrics to add, pop, commit and compact methods.
Only compact on startup. Regular compact seems to be flaky for now.
Add message_cache_commit_interval and message_cache_off_heap to config.
Remove hardcoded commit interval and use config value.
Add buildMessageInput() to InputService interface.

This allows the creation of a MessageInput object without initializing
it.
Better background thread name for the DiskJournalCache.
Use DiskJournalCache if message_cache_off_heap is set to true.
Add message cache settings to the default graylog2.conf.