When the query cache is disabled, the server shouldn't attempt to take the
query cache mutex.
By using the command line option --query_cache_type=0, the user can disable
(backport from mysql-pe)
mysql-test/t/query_cache_disabled-master.opt:
* added test case for bug38551
mysql-test/t/query_cache_disabled.test:
* added test case for bug38551
sql/set_var.cc:
* Added before-trigger to verify that query_cache_type wasn't turned off or on during
runtime.
sql/set_var.h:
* Changed order on how the enumeration is processed. By first projecting the
character representation of the variable to a temporary integer we can have
one function instead of two to check if the value is valid.
sql/share/errmsg-utf8.txt:
* Added error message for query cache disabled state
sql/sql_cache.cc:
* If the query cache is disabled at start up, shorten the execution path and avoid
grabbing the query cache mutex each time the invalidate interface methods are called.
sql/sql_cache.h:
* Added new methods to set the query cache into a disabled state.
