Speed up EntrySortIterator by 20%.

[jeschkies]

What this PR does / why we need it:
This patch replace the underlying heap of the EntrySortIterator. Instead of using a min or max heap the entry iterators are sorted at the beginning. Once the iteration starts only the first entry iterator is re-positioned depending on its current value. This will yield the same runtime complexity as the current heap based solution. However, since the new algorithm accesses the first element it can be cached. Thus the number of access calls to Entry().Timestamp are halved.

› benchstat before.txt after.txt
name                        old time/op  new time/op  delta
SortIterator/merge_sort-16  4.40ms ± 4%  4.38ms ± 3%     ~     (p=0.400 n=9+10)
SortIterator/sort-16        3.65ms ± 2%  2.80ms ± 3%  -23.25%  (p=0.000 n=9+10)

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

[cyriltovena]

Looking good please make sure you correctly merge freshly change there.

[cyriltovena]

LGTM