Get rid of using namespace std in src/rpc

Summary:
as described
take no prisoners

Test Plan:
make check
../qa/pull-tester/rpc-tests.py

Reviewers: #bitcoin_abc, deadalnix, CCulianu

Reviewed By: #bitcoin_abc, deadalnix, CCulianu

Subscribers: CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D438