Auto merge of #1919 - ebfull:abstract-verification, r=str4d

Isolate verification to a ProofVerifier context object that allows verification behavior to be tuned by the caller.

This is an alternative foundation for #1892, i.e., #1892 will have to be changed if this PR is accepted.

I think this is a safer approach because it allows us to isolate verification behavior to a single object. This will come in handy when @arielgabizon finishes the batching code.

src/wallet/rpcwallet.cpp

@@ -2707,7 +2707,10 @@ Value zc_raw_joinsplit(const json_spirit::Array& params, bool fHelp)
                          vpub_old,
                          vpub_new);
 
-    assert(jsdesc.Verify(*pzcashParams, joinSplitPubKey));
+    {
+        auto verifier = libzcash::ProofVerifier::Strict();
+        assert(jsdesc.Verify(*pzcashParams, verifier, joinSplitPubKey));
+    }
 
     mtx.vjoinsplit.push_back(jsdesc);