storage/engine: rename Iterator.{Seek,SeekReverse} to {SeekGE,SeekLT}

[nvanbenschoten]

This addresses a comment from #42210.

The commit also adjusts the reverse seek (now SeekLT) to be exclusive instead of
inclusive. This matches the API exposed by Pebble and eliminates the need for
the bug fix in 79b4c77.

[cockroach-teamcity]

This change is 

[petermattis]

thanks for doing this.

The change mostly looks rote. Besides the spanset.Iterator note, was there anything worth special attention here.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @petermattis)

[nvanbenschoten]

Besides the spanset.Iterator note, was there anything worth special attention here.

The only other parts to pay attention to are:

1. the implementation of rocksDBIterator.SeekLT, which can no longer use r.reseek.
2. StateLoader.LoadLastIndex, which should still be fine because we're never going to get to Raft log index MaxUint64 (and if we did, we'd have bigger problems) so we can consider it exclusive instead of inclusive.

bors r=petermattis

[petermattis]

StateLoader.LoadLastIndex, which should still be fine because we're never going to get to Raft log index MaxUint64 (and if we did, we'd have bigger problems) so we can consider it exclusive instead of inclusive.

This is the sole use of SeekLT in the code base currently. Your pebbleMVCCScanner refactor will add another one. I'm wondering if we should try to remove SeekLT from the Iterator interface.

[craig]

Build succeeded

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[nvanbenschoten]

This is the sole use of SeekLT in the code base currently. Your pebbleMVCCScanner refactor will add another one. I'm wondering if we should try to remove SeekLT from the Iterator interface.

I don't have strong feelings about this. It's nice to have access to SeekLT and it doesn't weigh down the interface too much, especially because we already have SimpleIterator. I'll defer to your judgment.