A read-only or cloud-tiered volume loads a SortedFileNeedleMap, which held
both its .idx and its .sdx open for the life of the process. On a server with
~600K tiered volumes that is 1.2M descriptors before a single read, enough to
exhaust the fd limit and take the listeners down. The .dat is not the problem:
a tiered volume serves it from the remote backend.
Neither index file is needed except while a lookup is in flight, so borrow them
from a bounded process-wide pool instead. An idle volume now holds zero
descriptors; a busy one keeps its handles hot rather than paying an open() per
needle. Reads borrow O_RDONLY, so a volume on a read-only mount answers lookups
that previously failed at load. Sync tracks whether a tombstone was appended,
which also drops the fsync-per-volume storm at shutdown.
