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
https://github.com/zcash/zcash/pull/2652/commits/4699d0eb36e2049f692ead9f44c5886ba87a007c

diff --cc src/init.cpp
Simple merge

diff --cc src/test/rpc_wallet_tests.cpp
Simple merge

diff --cc src/wallet/rpcwallet.cpp
Simple merge