[tzdb] Replace shared_mutex with mutex. (#87929)
The overhead of taking a std::mutex is much lower than taking a reader
lock on a shared mutex, even under heavy contention.
The benefit of shared_mutex only occurs as the amount of
time spent in the critical sections grows large enough.
In our case all we do is read a pointer and return the lock.
As a result, using a shared lock can be ~50%-100% slower
Here are the results for the provided benchmark on my machine:
```
2024-04-07T12:48:51-04:00
Running ./libcxx/benchmarks/shared_mutex_vs_mutex.libcxx.out
Run on (12 X 400 MHz CPU s)
CPU Caches:
L1 Data 32 KiB (x6)
L1 Instruction 32 KiB (x6)
L2 Unified 1024 KiB (x6)
L3 Unified 32768 KiB (x1)
Load Average: 2.70, 2.70, 1.63
---------------------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------------------
BM_shared_mutex/threads:1 13.9 ns 13.9 ns 50533700
BM_shared_mutex/threads:2 34.5 ns 68.9 ns 9957784
BM_shared_mutex/threads:4 38.4 ns 137 ns 4987772
BM_shared_mutex/threads:8 51.1 ns 358 ns 1974160
BM_shared_mutex/threads:32 57.1 ns 682 ns 1043648
BM_mutex/threads:1 5.54 ns 5.53 ns 125867422
BM_mutex/threads:2 15.5 ns 30.9 ns 21830116
BM_mutex/threads:4 15.4 ns 57.2 ns 12136920
BM_mutex/threads:8 19.3 ns 140 ns 4997080
BM_mutex/threads:32 20.8 ns 252 ns 2859808
```
GitOrigin-RevId: 11f22f1a963ab3c8949cb723a63c07d7a409c8a8
3 files changed
