Change "sincedb_clean_after" defaults

[fbaligand]

The new option sincedb_clean_after is great !
But I would appreciate some slight changes :

• define the value in seconds, as it is the case for all the 5 other options that deal with a duration.
• set no default value. It allows to have same behavior for people that just update Logstash and implies no bug if start_option => begininng is set and no ignore_older. It gives also the ability to not activate the option for Logstash users who don't want sincedb auto-cleanup. Endly, it allows to have a consistent default behavior between ignore_older and sincedb_clean_after options, that are tied.
• precise in the documentation that this option should be higher than ignore_older option, to have a consistent behavior.