PostgreSqlSliceStore: fix conflict set duplication
SolutionGuarantor: constraint to one continuation per slice

The existing continuation constraint ensured an equal number of segments
are chosen for each side of a slice, but allowed more than one such pair
to be selected. Constrain so that no more than two segments involving
any single slice can be selected (i.e., at most one left segment and at
most one right segment). See the diagnostic query in catsop/catsop#39:

https://github.com/catsop/catsop/issues/39#issuecomment-59403795