Relayer load test

[polymorpher]

See issues discussed in #259 . The load test should repeatedly simulate a reasonable amount of users creating wallets and transferring funds at roughly the same time, using REST endpoints provided by the relayer (as opposed to making smart contract calls directly). A report should be generated at the end of the test on success rates of each relayer API, each user, overall success rates, and error statistics (occurrence of each error types). The tests should be configurable, and performed on both testnet and mainnet.