We need a general purpose way for plugins to store state (on disk, es.. )

A few plugins need to keep information about the current state of the plugin, like `what is the lastest file I have read`,`which ID`, each plugins implement their own datastore and often use the current file system. (ref https://github.com/elasticsearch/logstash/blob/master/lib/logstash/inputs/s3.rb#L257)

Logstash should provide a pluggable backend for plugins developers to store this kind of information. A simple persistent key=value store would cover most cases.