This patch adds support of RENAME INDEX operation to the ALTER TABLE
statement. Code which determines if ALTER TABLE can be done in-place
for "simple" storage engines like MyISAM, Heap and etc. was updated to
handle ALTER TABLE ... RENAME INDEX as an in-place operation. Support
for in-place ALTER TABLE ... RENAME INDEX for InnoDB was covered by
MDEV-13301.

Syntax changes
==============

A new type of <alter_specification> is added:

<rename index clause> ::= RENAME ( INDEX | KEY ) <oldname> TO <newname>

Where <oldname> and <newname> are identifiers for old name and new
name of the index.

Semantic changes
================

The result of "ALTER TABLE t1 RENAME INDEX a TO b" is a table which
contents and structure are identical to the old version of 't1' with
the only exception index 'a' being called 'b'.

Neither <oldname> nor <newname> can be "primary". The index being
renamed should exist and its new name should not be occupied
by another index on the same table.

Related to: WL#6555, MDEV-13301