REBUILDS, 5.7.23 OR LATER 5.7 RLSES

ANALYSIS:
=========
If a table containing keys on varchar column(s) is created
prior to 5.7.23, after upgrade to higher 5.7 releases,
those keys are rebuilt even on a simple ALTER statement to
add a COMMENT to the table.

If 'PACK_KEYS' table option is not specified, indexes on
columns with string types are marked for packing through
'HA_PACK_KEY' flag. 'PACK_KEYS' is supported only for
MyISAM tables. Before 5.7.23, indexes on columns with
length greater than 8 bytes were marked for packing
irrespective of the storage engine. In 5.7.23 and higher
releases, the key flags 'HA_PACK_KEY' or
'HA_BINARY_PACK_KEY' are not set if the storage engine
doesn't support it.

In the bug scenario, a table containing a column of string
type with length more than 8 bytes is being created in
5.7.22. Thus the key flag 'HA_PACK_KEY' is set. After
upgrade to higher 5.7 releases, a simple ALTER statement
to change the table COMMENT using INPLACE algorithm is
executed.

For INPLACE algorithm, we compare the changes between the
original and new table definitions. The result of this
comparison is then passed to SE which determines whether it
can carry out these changes in-place. For keys, we check
if the key types are compatible between old and new
tables, in 'has_index_def_changed()'. This check finds that
the key information of the old table has 'HA_PACK_KEY' set
thus inferring that the index has changed. This information
is then passed to the SE which recreates the index.

FIX:
====
The flags 'HA_PACK_KEY' or 'HA_BINARY_PACK_KEY' are now
removed depending on the SE capabilities, while preparing
the table share for the source table.

Change-Id: I51bd34d04e22b549392ac76411fe64798e8fa96f