Fix SharedLock ThreadLocal threadWriters memory leak may cause CPU usage to reach 100%

[bandalgomsu]

This PR SharedLock.decrementWriters() to call threadWriters.remove() when the per-thread writer depth reaches zero, preventing unused ThreadLocal entries from lingering.
This helps avoid the ThreadLocalMap.expungeStaleEntry() CPU spikes reported in #3489

Thanks !

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[a-TODO-rov]

This might actually work. @tishun what do you think ?

[a-TODO-rov]

Actually after second look and brief discussion with @tishun , there are some problems with that solution.
The main problem is same thread can go between 0 and 1 threadWriters very frequently in small interval of time, which creates a constant spike of ThreadLocal$SuppliedThreadLocal , which could lead to OOM error.

[bandalgomsu]

Thanks for your feedback 🙂
I agree your comment

Then, what do you think about reducing the remove() frequency based on probability or an idle count threshold ??