ALTER TABLE AND/OR PLUGIN/SEMISYNC

If a plugin was uninstalled, thread local values for plugin
variables of string type with PLUGIN_VAR_MEMALLOC flag were
not freed.

With this patch these variables are freed when thread is
done (like all other variables).

sql/sql_class.h:
  Added variable which stores memory hunks allocated
  for PLUGIN_VAR_MEMALLOC values.
sql/sql_plugin.cc:
  Normally all memory allocated for dynamic variables values must
  be freed by cleanup_variables().
  
  But if a plugin was uninstalled, descriptors of it's system
  variables are lost. Still some memory may be occupied for thread
  local values. It is ok for most kinds of variables, as they're
  stored on dynamic_variables_ptr and freed when thread is done.
  
  Values for PLUGIN_VAR_MEMALLOC variables are stored separately.
  These lost values are handled by plugin_var_memalloc_free().