core: use Load and Delete separately for sync.Map

sync.Map.LoadAndDelete is Go 1.15+ only

core: add note to 'other call in progress' check

bump version to 2.0.28

bump version to 2.0.27

bump version to 2.0.26

bump version to 2.0.25

bump version to 2.0.24

bump version to 2.0.23

bump version to 2.0.22

bump version to 2.0.21