Summary:
See new atomic.h file comments for motivation.
I have updated HyperClockCache to use the new atomic wrapper, fixing a few cases where an implicit conversion was accidentally used and therefore mixing std::memory_order_seq_cst where release/acquire ordering (or relaxed) was intended. There probably wasn't a real bug because I think all the cases happened to be in single-threaded contexts like constructors/destructors or statistical ops like `GetCapacity()` that don't need any particular ordering constraints.
Recommended follow-up:
* Replace other uses of std::atomic to help keep them safe from bugs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12051
Test Plan:
Did some local correctness stress testing with cache_bench. Also triggered 15 runs of fbcode_blackbox_crash_test and saw no related failures (just 3 failures in ~CacheWithSecondaryAdapter(), already known)
No performance difference seen before & after running simultaneously:
```
(while ./cache_bench -cache_type=fixed_hyper_clock_cache -populate_cache=0 -cache_size=3000000000 -ops_per_thread=500000 -threads=12 -histograms=0 2>&1 | grep parallel; do :; done) | awk '{ s += $3; c++; print "Avg time: " (s/c);}'
```
... for both fixed_hcc and auto_hcc.
Reviewed By: jowlyzhang
Differential Revision: D51090518
Pulled By: pdillinger
fbshipit-source-id: eeb324facb3185584603f9ea0c4de6f32919a2d7
