Workaround for incremental unsat cores (#1962)

This commit implements a workaround for computing unsat cores while
doing incremental solving to address #1349. Currently, our resolution
proofs do not handle clauses with a lower-than-assertion-level correctly
because the resolution proofs for them get removed when popping the
context but the SAT solver keeps using them. The workaround changes the
behavior of the SAT solver to add clauses always at the current level if
incremental solving and unsat cores are enabled. This makes sure that
all learned clauses are removed when we pop below the level that they
were learned at. This may degrade performance because the SAT solver has
to relearn clauses.

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