Fix subtlety with SDPOR/ODPOR initials
Computing initials for SDPOR/ODPOR requires
extending a search with respect to
a prefix of an execution. Previously, events
added to the extension were referred to by
their location within the larger execution
from which the prefix was taken. The problem
is that the new events need to be referred
to by their position relative to the
*extension* of the prefix; otherwise,
asking about "happens-before" in the extension
while using their handles in the full execution
will give incorrect results
