Cleanup producer_plugin pimpl idiom.

[greg7mdp]

Normally, for the pimpl idiom, the implementation resides in the private class, in our case producer_plugin_impl.

Before this change, many functions were defined in producer_plugin, directly accessing fields from producer_plugin_impl via the my pointer.

This PR does not change what the code does, it should be 100% equivalent to what was there before, however it moves most of the member functions implementation within the producer_plugin_impl class.

I wouldn't mind doing a even fuller cleanup, so that all the producer_plugin methods forward to producer_plugin_impl and are grouped together at the bottom of the file, but it might make the change even harder to follow. Please let me know what you think.

PS: most of the changes are white space changes, to make formatting consistent within this file. You make benefit from appending ?w=1 to the diff window url to limit whitespace differences.