HDDS-16350. Avoid a point-get for the start-key check in RDBTable.getRangeKVs

[rich7420]

What changes were proposed in this pull request?

RDBTable.getRangeKVs decided whether startKey exists with get(startKey) == null,
a full point-get that materializes and then discards the value byte[], immediately
before it.seek(startKey) lands the iterator on that same key.

seek() already positions the iterator and returns the landing entry without consuming
it (the range loop below still starts from that entry), so the existence check can reuse
the landing entry instead of a separate get: startKey is present iff the landing key
equals startKey.

Before:

if ((prefix == null || startKey.length > prefix.length)
    && get(startKey) == null) {
  // Key not found, return empty list
  return result;
}
it.seek(startKey);

After:

final KeyValue<byte[], byte[]> seeked = it.seek(startKey);
if ((prefix == null || startKey.length > prefix.length)
    && (seeked == null || !Arrays.equals(seeked.getKey(), startKey))) {
  // start key not found, return empty list
  return result;
}

This drops one point-get per getRangeKVs call plus the value byte[] it allocated only
to discard. Behavior is unchanged: the loop still starts at the seek() landing entry, and
the empty-result path fires on exactly the same inputs.

A JMH microbenchmark over a real RocksDB (100k keys, 512-byte values, present start key),
old (extra get) vs new (seek landing check), average time and allocation per call:

page	time old -> new	alloc old -> new
1	1.62 -> 0.99 us	1136 -> 640 B/op
10	3.86 -> 3.39 us	6176 -> 5680 B/op
100	27.6 -> 25.6 us	56576 -> 56080 B/op
1000	254 -> 250 us	560581 -> 560085 B/op

The saving is exactly one point-get: a constant ~496 B/op and ~0.5-0.6 us per call, so it is
largest as a fraction on small pages (~39% at page 1) and shrinks toward the iteration cost
as pages grow.

What is the link to the Apache Jira

https://issues.apache.org/jira/browse/HDDS-16350

How was this patch tested?

https://github.com/rich7420/ozone/actions/runs/33313659945

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