QUERY CACHE - add STATISTICS per query and show this informations in qc_info plugin

Change sql_cache.cc and sql_cache.h to add hit rates and statistics
usable with qc_info plugin

Statistics:
total hits / total time expent in query cache
hits in three categories:
1) low period category: period between the last hit and the current hit < query expent time
2) high period category: period < query expent time * 10 and > query expent time
3) outlier category: period > 1 second, and period > query expent time * 10
3.1) in a future version instead of * 10, we could use number of competitors (concurrent) when inserting this query at query cache
for example if we have 9 concurrent inserts of the same query at query cache this number *10 is an acceptable value for high period, if we have n concurrent inserts a (n+1) value is acceptable

These categories helps to understand what kind of query we have cached:
1) low period hits: in this case if we lose this query entry, we will have a performace problem, and users will tell "the server is slow now"
2) high period hits: we have a query that is executed with a good frequency, removing it we will have a higher i/o in disks
3) outlier period hits: this query isn't a query that we should cache, or we can loose it and we don't see users reporting "hey the server is slow, or the disks is spinning a lot"

this allow a better query cache control (MDEV-4584) and a better cache hit rate when optimizing queries with SQL_CACHE/SQL_NO_CACHE

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a second part could be an variable (query_cache_stats=0/1) to turn off/on the statistics and use a block memory just to statistics instead of one big block sharing memory with queries flags

in this case:
turn on -> off, should clean this stats memory block
turn off -> on, should create stats with 0 value (maybe a problem with lock contention), or clean all query cache and start from zero

changing query cache queries (invalidate, table invalidate, reallocation, etc) should handle the stats block too

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tables of this plugin:

query_cache_queries:

query_cache_queries_tables:

query_cache_tables