Avoid Stripe Mutex lock contention for RWW

[masaori335]

I found that if OpenDir has own reader-writer lock, it doesn't need StripeSM mutex. This means we can avoid the StripeSM mutex lock contention issue for reader-while-writer cases.

Benchmarking RWW is a bit tricky but one of benchmark says max rps is improved 9.9% in below conditions.

Conditions:

• 10 urls
• plaintext http
• response body size: 256 bytes
• origin returns Cache-Control: public, max-age=0 ///< some requests goes RWW path
• 63 exec_thread
• 40 stripes (8 disks x 5 volumes)

Result:

• vanilla: 58,220.9 req/s
• patch: 63,999.7 req/s

part of #12788