Correctly saves the buffer capacity in BufferedRowReaderIterator

[cyriltovena]

I was looking at compaction and trying to figure why it was slow.

❯ benchstat before.txt afte.txt
name                  old time/op    new time/op    delta
BufferedRowReader-16     709ns ±14%     306ns ± 3%   -56.88%  (p=0.008 n=5+5)

name                  old alloc/op   new alloc/op   delta
BufferedRowReader-16    2.72kB ± 0%    0.00kB       -100.00%  (p=0.008 n=5+5)

name                  old allocs/op  new allocs/op  delta
BufferedRowReader-16      1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

This is most likely makes flushing a block 50% faster and lighter.

PS: Not fully sure why we loose the slice capacity when slicing.

[simonswine]

LGTM

[simonswine]

That old line surely reduced the capacity by-1

It is only when you cut of the end that the capacity stays the same, so buf[:cap(buff)-1], will still have the same capactity.

[cyriltovena]

The PR is ready to merge but it trigger another bug hidden in the merge. I'll update this PR to have the fix soon.