Plugins: Notify before Checkpoint
Change Comment Notify/Checkpoint
Plugin System: Remember Last Checkpoint/Notify

Adds an internal state and get/set methods to the IPlugin
and INotify classes that are updated in the PluginConnector
after `notify(...)` and `checkpoint(...)` calls.
Constify Get Methods
Merge pull request #541 from ax3l/topic-pluginNotifyBefCP

Plugins: Notify before Checkpoint