[BUG](common): ARCCache's param c means cache capacity

[zztaki]

Describe the bug (描述bug)
ARCCache's param c means cache capacity, but in the code, it is not. So this will cause ARCCache's real capacity will be maxCount/2.

To Reproduce (复现方法)

Expected behavior (期望行为)

Versions (各种版本)
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Additional context/screenshots (更多上下文/截图)

[zztaki]

If you think this makes sense, I am glad to submit a PR and add some tests for ARCCache :)

[zztaki]

[zztaki]

Maybe you consider BList overhead. But in fact, BList doesn't store Value and only stores some indexes, which represent some recently visited objects that may be frequent. :)

curve/src/common/arc_cache.h

Lines 90 to 92 in bfd5acb

	struct BListVal {
	bmap_iter map_iter;

[wuhongsong]

#2852 @zztaki this pr will be ok.may reopen it later.

[skypexu]

#2962