Avoid a race within edge::get_datum
Avoid a race within edge::get_datum

Two get_datum calls can race and both get the same datum before either
pop happens.

linus-sherrill/sprokit,mathstuf/sprokit,Kitware/sprokit,mathstuf/sprokit,linus-sherrill/sprokit,linus-sherrill/sprokit,mathstuf/sprokit,mathstuf/sprokit,Kitware/sprokit,Kitware/sprokit,Kitware/sprokit,linus-sherrill/sprokit