CLOSE - query cache expent time searching query

should be nice know how many time we "lost" in query cache searching a query to tune query cache better
1) total expend time with hits
2) total expend time without hits
1+2) total expend time of query cache search

we could do this per qc query too
when we read a query from cache, we add +1 to query hit counter (using last patch that i sent in mdev-4581)
we could add the time expend to read query from cache to this entry to a total time var

after let's say +- 100hits, we can check if the query cache is faster than executing the query, ex:
query cache: 100hits, 500ms , ~500ms/100hits = 5ms/hit for this entry
expend time from this entry (if we execute the query again): 1ms - this information is in qc entry with the mdev-4581 patch

if expend query time to execute query is slower we could "mark" (just a math x>y) this query entry to be deleted first in a low memory situation

this can be a new performace information (slow query cache entries) to know if our query cache is doing a god or a bad work too

with this information we could tune better with MDEV-4588 with a per tables max queries in cache