Resolve misconception with SDPOR pseudocode impl.

Resolve misconception with SDPOR pseudocode impl.

This commits fixes a semantics mismatch between
the implementation and the pseudocode in SDPOR.
The trouble was that we didn't stop searching
for an initial when we noticed that ANY initial
was contained in the backtrack set; instead, we
simply searched until we found ANY initial contained
OUTSIDE OF the backtrack set. The latter is not
equivalent to the former, so SDPOR was doing
more work than necessary.

A similar implementation issue is also present in
ODPOR which will be resolved in due time.