Note for reviewers: txmempool.* changes are not necessary since we've already renamed them to more appropriate names.

Please still include the change in test_bitcoin.cpp. Although it appears useless, there's no telling if Core will write a new test in the future that depends on this, which we would then want to backport. Essentially, by leaving this change out, we are taking ownership of the code. Unless we also want to own net.h's definitions and switch vNodes to a vector of smart pointers, then I don't think this half-ownership is wise.