Implement lazy proof checking modes (#7106)

Implement lazy proof checking modes (#7106)

This implements several variants of lazy proof checking in the core proof checker.

Note this extends the ProofNode class with an additional Boolean d_provenChecked indicating whether the d_proven field was checked by the underlying proof checker.

This PR updates the default proof checking mode to lazy. The previous default can now be enabled by --proof-check=eager-simple.