WL#5252: Deprecate --optimizer_search_depth=63

Deprecate the specific value 63 for the variable optimizer_search_depth.
According to Server PT decision as of 2010-02-10, the value will be deprecated 
in release 5.5, and eventually removed in the next major release after that.

Deprecate the specific value 63 for optimizer_search_depth, which enables the 
old optimizer behavior. choose_plan() will in this case call find_best() and 
not greedy_search().

The documentation for choose_plan() contains the following note:
"'MAX_TABLES+2' denotes the old implementation of find_best before
the greedy version. Will be removed when greedy_search is approved."

greedy_search() has been in the code base since 5.0, so it is about time to 
deprecate this feature now.

Using this value is documented in the help code for the optimizer_search_depth 
variable.

It is also documented in the user manual: "If set to the maximum number of 
tables used in a query plus 2, the optimizer switches to the algorithm
used in MySQL 5.0.0 (and previous versions) for performing searches", but this 
paragraph is slightly confusing (is it referring to the maximum number of 
tables that can be used, or the number of tables being used, and what is then 
this value?).

See also mail threads:
"Should we remove find_best()?" sent to mysql_dev_optimizer by Øystein Grøvlen 
at 2009-12-04, and
"why is the default of --optimizer_search_depth 62?" sent to 
internal (http://lists.mysql.com/internals/37631) by Guilhem Bichot at 
2010-01-12.

Related bug cases:

BUG#50052 reports a regression in 6.0, --optimizer_search_depth=63 crashes. We 
can close this as "will not fix" if we deprecate this behavior in 5.1 and 5.5, 
and remove it in 6.0 (presuming that 6.0 will be the next major release after 
5.5).

Related tests:

greedy_optimizer - checks query cost for old and greedy cost-based optimizer 
subselect3 - tests that old optimizer works for semijoin conversion of subquery
optimizer_search_depth_basic - Checks legal values for variable

Deprecation of server features follow the approved policy at: https://
inside.mysql.com/wiki/DeprecatingServerFeatures

Related worklogs:

WL#5265: Remove deprecated 5.5 features

Implement deprecation by adding an update function to the system variable 
optimizer_search_depth and issue a warning (using WARN_DEPRECATED()) when the 
value 63 is given.