Remove unnecessary copies from PR #497

lib/NegativeAcksTracker.cc

    auto ledgerMap = it->second;
    for (auto ledgerIt = ledgerMap.begin(); ledgerIt != ledgerMap.end(); ++ledgerIt) {
        auto entrySet = ledgerIt->second;

This makes a copy. Is this intended to be a reference instead - auto& entrySet = ledgerIt->second;?
This line also makes a copy `auto ledgerMap = it->second;
Originally posted by @arhoads in #497 (comment)

Motivation

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing tests.

Documentation

• doc-required
  (Your PR needs to update docs and you will update later)

• doc-not-needed
  (Please explain why)

• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)