Single connection option

[yifanguan]

Add a new command-line option: --single-transaction to examine RPC requests of a single random transaction.

Usage:
./tpccbenchmark --execute=true --single-transaction

[robertsami]

@yifanguan this looks useful, but can you separate it from the RETURNING... clause changes and keep this PR scoped?

also, can we add an option to specify the the transaction you run? if not specified, maybe we run a random one, but otherwise we run the one specified?

[yifanguan]

Rebase the PR to make it scoped.

I can definitely add an option to specify which transaction to examine (which HikariDataSource to get a connection) using nodes numbers (1, 2, 3, etc).
e.g. ./tpccbenchmark --execute=true --single-transaction 1
However, I'm not sure whether this can help us understand RPC requests better. Each time transactions are dynamically scheduled, I feel like this is random scheduling. Specifying a specific node number may not be helpful. Please correct me if I misunderstand anything.