Introduce skeleton plugin process.

For external parsing there will be a separate process that handles
parse requests sent from MaxScale. From MaxScale's perspective there
will just be a specific query classifier plugin that upon startup
launches that external process and manages its lifetime. That is,
MaxScale proper will be completely oblivious of this arrangement.

The skeleton process introduced here does nothing; just sits in
a loop sleeping. The skeleton query classifier plugin currently
loads the MySQL embedded query classifier and performs parsing
using it. That way the external process based query classifier can
immediately be used.

Next step is to introduce process management into the picture without
actually moving the parsing anywhere.

query_classifier/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_subdirectory(qc_mysqlembedded)
+add_subdirectory(qc_pp)
 
 if(BUILD_TESTS)
   add_subdirectory(test)