[#4585] Addressed review comments

modified:   changelog_unreleased/4585-client-identifier-differences-results-in-parallel-execution-of-chg_add-and-chg_remove-for-same
modified:   src/bin/d2/d2_update_mgr.cc
modified:   src/bin/d2/d2_update_mgr.h
modified:   src/bin/d2/tests/d2_update_mgr_unittests.cc
modified:   src/lib/d2srv/nc_trans.cc
modified:   src/lib/d2srv/nc_trans.h