WL#1960: Pluggable Storage Engines

Implement Loadable Storage Engines   
  

 
 

Storage Engines may be loaded by the the user (with appropiate privilege   
checks) with the following commands:   
 
INSTALL PLUGIN  SONAME "filename" 
 
(note, these are the commands introduced by the plugin framework, 
no new syntax was created for this worklog) 
 
As part of the work, the CSV and Example storage engines were altered to 
be plugable as per the MySQL plugin framework abi. 
 
The code now relies on the plugin framework to enumate builtin and loaded 
storage engines. 
 
Currently, unloading storage engines at run time is not advised. 
 
 

Made use of the existing plugin framework. Simply used handlerton as 
storage engine specific plugin info. 
 
As part of the work, enum db_type was deprecated with preference to the 
handlerton 
 
The plugin framework was extended to permit built-in plugins and for the 
storage engine to use the plugin framework to enumerate all handlertons.