Simplify BUFFER_COUNT in ConcurrentLruCache to a constant

[s-chan-o]

Problem

The detectNumberOfBuffers() method in ConcurrentLruCache.ReadOperations attempts to scale the read buffer count based on the number of available processors:

private static int detectNumberOfBuffers() {
    int availableProcessors = Runtime.getRuntime().availableProcessors();
    int nextPowerOfTwo = 1 << (Integer.SIZE - Integer.numberOfLeadingZeros(availableProcessors - 1));
    return Math.min(4, nextPowerOfTwo);
}

However, the use of Math.min(4, nextPowerOfTwo) caps the result at 4 for any system with 3 or more processors. For systems with 5+ processors, nextPowerOfTwo would be 8 or more, but the Math.min always returns 4. The CPU-detection logic adds complexity without any actual effect for the vast majority of deployment environments.

As noted in gh-36780, the value is "basically always just 4".

Fix

Simplify BUFFER_COUNT to a plain constant 4, removing the detectNumberOfBuffers() method entirely. This is consistent with the feedback from gh-29520 that informed the original cap at 4.

Closes gh-36780

[bclozel]

@s-chan-o please sign off your commit, we cannot accept this contribution without it. Thanks

[s-chan-o]

@bclozel Added Signed-off-by in 592ab02. Thanks!

[bclozel]

Thanks for your contribution @s-chan-o !