wfstack API: rename cds_wfs_first_blocking to cds_wfs_first

cds_wfs_first never needs to block. This operation can be used to check
if the stack returned by pop_all is empty or not, so it is quite
interesting to have a fully non-blocking semantic for all of
enqueue/pop_all/first operations. Only cds_wfs_next may block.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>