Track the current sequence of states with UDPOR
Prior to this commit, we only tracked the "current"
state that was visited by the program. This is not
sufficient however: we must also keep track of the
sequence of states that UDPOR visited to *generate*
the configuration it is currently looking at. This
is required since we need to be able to generate
a fresh remote process when UDPOR decides to re-explore
a configuration `C` in its second recursive call
if it determines that an alternative exists. The
state regeneration is very similar (practically
identical) to that which is performed in the DFSChecker.
