[tm-bench] Incorrectly computes metrics

[ebuchman]

For one, we use the BlockchainInfo rpc call which returns max 20 blocks at a time, so we need to check that and keep querying if there are more blocks.

Also we could be more clear about the computation: just we know how many txs are sent, we should find the first block with a tx and keep grabbing blocks until we've committed enough txs, then look at the timestamp difference between that first and last block.

I started some WIP fixes here: #100