Measure average transaction commit time

Right now, [we don't care about RPC responses](https://github.com/interchainio/tm-load-test/blob/3c880773563afac4fa11da0eaa509b4751a12798/pkg/loadtest/transactor.go#L143), and we do need to build in a way to measure the average time it takes to commit a transaction.

We should measure the impact of this on the load testing performance and, if it affects performance, only measure commit times for a subset of the transactions sent through.