perf(algo): single memory allocation in NewCountMinSketch

[arturmelanchyk]

Description

Please explain the changes you made here.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• For public APIs, new features, etc., PR on
  docs repo staged and linked here

Instructions

• The PR title should follow the Conventional Commits
  syntax, leading with fix:, feat:, chore:, ci:, etc.
• The description should briefly explain what the PR is about. In the case of a bugfix, describe or
  link to the bug.
• In the checklist section, check the boxes in that are applicable, using [x] syntax.
  • If not applicable, remove the entire line. Only leave the box unchecked if you intend to come
    back and check the box later.
• Delete the Instructions line and everything below it, to indicate you have read and are
  following these instructions. 🙂

Thank you for your contribution to Dgraph!