WL#9817: Change default query_cache_size to zero

Affects: Server-8.0   —   Status: Complete

In MySQL 5.6 we changed the default query_cache_size to 1M, with query_cache_type 
to OFF.  From a usability perspective, this made sense: having to enable two 
options together to work is confusing.

pfs shows that the Query cache mutex is still acquired when the size is non-zero:

Suggested default for 8.0 is zero.

User Documentation

Functional Requirement

Default value of variable 'query_cache_size' at server start should be : 0 (zero)
The worklog changes the default value of variable :

query_cache_size  -   0       ( new default value )
                      1048576 ( old default value )

Some discussions about Query Cache:
From Links 
 - https://bugs.mysql.com/bug.php?id=65336 and
 - http://dev.mysql.com/doc/refman/5.7/en/query-cache-configuration.html

"If the server is started with query_cache_type set to 0, it does not acquire
the query cache mutex at all, which means that the query cache cannot be enabled
at runtime and there is reduced overhead in query execution."

Changing the setting while the server is running does reduce the cost of the
query cache because queries aren't cached and much checking time is reduced but
there is still overhead compared to having it off at startup. That overhead is
greater with row-based replication.

Summary :
Query cache mutex performs solely based on the value of query_cache_type
variable at the time of server startup. If query_cache_type is ON while
starting the server, changing the variable value at runtime will not reduce the
mutex overhead. The change in default value of query_cache_size will not affect
this behavior.