Transaction Priority Support

[HeCorr]

Is your feature request related to a problem? Please describe.
CockroachDB supports Transaction Priorities which allow a transaction to be executed before the others, but pgx doesn't seem to support it.

Describe the solution you'd like
I would like for pgx to support setting the transaction priority via pgx.TxOptions.

Describe alternatives you've considered
It should be possible to manually set the transaction priority after it has been created (BeginTx) with the SET TRANSACTION query but I haven't tried that.

Additional context
From what I can tell, Postgres does not support transaction priorities so it might be necessary to check which database pgx is connected to before applying the option (is that even possible though?).

[jackc]

I'm reluctant to add direct support to non-PostgreSQL features. In most cases it isn't an issue because those features are accessed via the query string passed to Query or Exec. I suppose that a BeginQuery string field added to pgx.TxOptions might be a solution. If that string is not empty then it is used to start the transaction instead.

[HeCorr]

I'm reluctant to add direct support to non-PostgreSQL features.

Understandable.

I suppose that a BeginQuery string field added to pgx.TxOptions might be a solution.

Yeah that sounds good enough :)

[jackc]

Done in 9a5ead9.