Added ExPendingCommandFactory2 and SimpleExPendingCommandFactory2 to
replace, respectively, ExPendingCommandFactory and
SimpleExPendingCommandFactory by supporting creation of only single
commands rather than a sequence of commands which need to wrapped around
in cumbersome ways to make a depth first execution.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>