* fix(volume): cap leveldb OpenFilesCacheCapacity per index DB (#9139)
The leveldb opt.Options for NeedleMapLevelDb / Medium / Large never
set OpenFilesCacheCapacity, so each leveldb instance defaulted to
goleveldb's 500. On servers with thousands of volumes, that ceiling
stacks across DBs and exhausts even high ulimits, starving WAL
rotation:
failed to write leveldb: open .../000006.log: too many open files
CompactionTableSizeMultiplier=10 already keeps the SST count low,
so a small per-DB cache is sufficient. Cap at 16 / 32 / 64 for the
small / medium / large variants so per-DB FD usage is bounded.
* storage: hoist leveldb FD-cap values into named constants
Per review feedback: replace the inline 16/32/64 literals with
LevelDb{,Medium,Large}OpenFilesCacheCapacity, and move the rationale
(why 500 is too high per-DB on busy servers, what the tradeoff is)
into a package-level comment so future readers see the memory vs.
performance picture at the constant declaration instead of inline.
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