Add transaction sub-sequencer

[brendanlong]

This ensures that if you start a transaction, other queries outside
of it have to wait for the transaction to finish.

For example, if you do:

let%bind _ = Pgx_async.with_transaction (...)
and _ = Pgx_async.execute ...

This will now guarantee that the execute happens outside of the
transaction (either before or after it, but not during).

[brendanlong]

I guess we probably also need the deadlock detection we added in Mssql: https://github.com/arenadotio/ocaml-mssql/blob/master/src/client.ml#L28

[gtrak]

These are things we might potentially want, but they need more work and testing to get over the finish line. Closing for now.