Reduce dict_sys mutex contention for read-only workload

• Currently, during read-only workload with high contention (say 1024 threads)
  dict_sys mutex contention rises as a top-contention (as reported by performance schema).

• read-only workload ideally shouldn't have anything to do with dict_sys mutex
  but optimizer needs stats and to read stat dict_sys mutex is taken.

• dict_sys is global mutex even though optimizer may request stat for a said table only.

• this contention is heavily visible when operating with higher scalability and especially
  with more numa nodes things get worse.

<60 secs ro workload> ...

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How to solve it?

• Probably check how dict_sys mutex could be relaxed for concurrent access.

• Marko suggested a better solution. Starting 10.6 there is table->lock_mutex.
  Given table level mutex it could help reduce the contention.

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Very early POC has show improvement in sysbench-ro workload qps by 5-7%.
The approach also helps support the numa-scalability goal.