Provide better information regarding memory usage by each plugin

I have implemented this issue: https://github.com/piwik/piwik/issues/6708
Also I have discussed this with issue author and it looks like good implementation of his idea. @mattab what you think? Should be more or this is enough for this PR? 
to get the memory usage can you try re-use the `Timer` object which has `getMemoryLeak()` etc. ?