Clean up the mysql_priv.h file

mysql_priv.h has become a crufty catch-all for developers who don't want to make the effort to create proper header files for class and function declarations. It's a beast that needs refactoring (and renaming to something that makes sense)

Some problems with it:

* It is in the drizzled/ directory and yet also features client-side declarations.
* Function and class declarations for specific functionality are just dumped willy-nilly into this file. They should be broken out into appropriate header files
* Remove dead code -- I've already found 3 or 4 functions that are declared and never used in the code...
* Actually comment what the includes in this file are including. Sometimes, due to the naming of header files, it's not at all apparent what a header is actually including
* Flatten the include header tree and dependencies