This task is to allow storage engines that can execute GROUP BY or
summary queries efficiently to intercept a full query or sub query from
MariaDB and deliver the result either to the client or to a temporary
table for further processing.

- Added code in sql_select.cc to intercept GROUP BY queries.
  Creation of group_by_handler is done after all optimizations to allow
  storage engine to benefit of an optimized WHERE clause and suggested
  indexes to use.
- Added group by handler to sequence engine and a group_by test suite as
  a way to test the new interface.
- Intercept EXPLAIN with a message "Storage engine handles GROUP BY"

libmysqld/CMakeLists.txt:
  Added new group_by_handler files
sql/CMakeLists.txt:
  Added new group_by_handler files
sql/group_by_handler.cc:
  Implementation of group_by_handler functions
sql/group_by_handler.h:
  Definition of group_by_handler class
sql/handler.h:
  Added handlerton function to create a group_by_handler, if the storage
  engine can intercept the query.
sql/item_cmpfunc.cc:
  Allow one to evaluate item_equal any time.
sql/sql_select.cc:
  Added code to intercept GROUP BY queries
  - If all tables are from the same storage engine and the query is
    using sum functions, call create_group_by() to check if the storage
    engine can intercept the query.
    - If yes:
       - create a temporary table to hold a GROUP_BY row or result
       - In do_select() intercept normal query execution by instead
         calling the group_by_handler to get the result
       - Intercept EXPLAIN
sql/sql_select.h:
  Added handling of group_by_handler
  Added caching of the original join tab (needed for cleanup after
  group_by handler)
storage/sequence/mysql-test/sequence/group_by.result:
  Test group_by_handler interface
storage/sequence/mysql-test/sequence/group_by.test:
  Test group_by_handler interface
storage/sequence/sequence.cc:
  Added simple group_by_engine for handling COUNT(*) and
  SUM(primary_key).  This was done as a test of the group_by_handler
  interface