MCOL-3593 Disabled full optimizer run and enabled copy-pasted simplify_joins.

Disabled 4th if block in buildOuterJoin to handle non-optimized MDB query
    structures.

    Broke getSelectPlan into pieces: processFrom, processWhere.

MCOL-3593 UNION processing depends on two flags isUnion that comes as
arg of getSelectPlan and unionSel that is a local variable in
getSelectPlan. Modularization of getSelectPlan broke the mechanizm.
This patch is supposed to partially fix it.

MCOL-3593 Removed unused if condition from buildOuterJoin that allows
unsupported construct subquery in ON expression.
Fixed an improper if condition that ignors tableMap entries w/o condition
in external_lock thus external_lock doesn't clean up when the query
finishes.
Fixed wrong logging for queries processed in tableMode. Now rnd_init
properly sends queryText down to ExeMgr to be logged.

MCOL-3593 Unused attribute FromSubQuery::fFromSub was removed.
 getSelectPlan has been modularized into: setExecutionParams,
 processFrom, processWhere. SELECT, HAVING, GROUP BY, ORDER BY
 still lives in getSelectPlan.
Copied optimization function simplify_joins_ into our pushdown
 code to provide the plugin code with some rewrites from MDB it
 expects.
The columnstore_processing_handlers_fallback session variable
 has been removed thus CS can't fallback from SH to partial
 execution paths, e.g. DH, GBH or plugin API.

MCOL-3602 Moved MDB optimizer rewrites into a separate file.

Add SELECT_LEX::optimize_unflattened_subqueries() call to fix IN
 into EXISTS rewrite for semi-JOINs with subqueries.

disable_indices_for_CEJ() add index related hints to disable
 index access methods in Cross Engine Joins.

create_SH() now flattens JOIN that has both physical tables and
 views. This fixes most of views related tests in the regression.

dbcon/mysql/ha_mcs.h

@@ -1,5 +1,6 @@
 /* Copyright (C) 2014 InfiniDB, Inc.
    Copyright (C) 2016 MariaDB Corporation
+   Copyright (C) 2019 MariaDB Corporation
 
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License