WL#2791: Query Cache information in EXPLAIN output

The output of EXPLAIN, or an alternative command, should show information about
whether the result of a query will be stored in the Query Cache.

The information provided could include whether or not the query is cacheable,
and if not it could give the reason (ie. refers to UDF, refers to user-defined
variable, refers to non-idempotent function, etc). It could also be shown
whether the query is already in the cache and how much space the result for the
query uses in the query cache.

Possible implementations include an additional column of output in EXPLAIN,
which could either be automatically included or triggered by EXPLAIN SQL_CACHE
SELECT or some other syntax. Another possibility is to show this output only if
it is asked for explicitly, and to show the output independent of any other
output.

Based on BUG#11870