Auto merge of #2670 - arielgabizon:lowmemprover, r=daira

Low memory prover

This PR integrates @ebfull 's low memory changes:  https://github.com/zcash/zcash/pull/2243/commits
on top of @str4d 's work bringing in libsnark as a subtree
4699d0eb36

src/wallet/rpcwallet.cpp

@@ -2543,12 +2543,6 @@ UniValue zc_benchmark(const UniValue& params, bool fHelp)
 
     std::vector<double> sample_times;
 
-    if (benchmarktype == "createjoinsplit") {
-        /* Load the proving now key so that it doesn't happen as part of the
-         * first joinsplit. */
-        pzcashParams->loadProvingKey();
-    }
-
     JSDescription samplejoinsplit;
 
     if (benchmarktype == "verifyjoinsplit") {