Add support for locking table in pgx-driver

[tsfcode]

A common pattern in postgres deployment is to use pgbouncer in transaction pooling mode. This is to support at large number of concurrent clients to work around the process based postgres connection model.
Unfortunately pgbouncer does not support advisory locks in transaction pooling mode, which is the most commonly used mode. To work around this we introduce this traditional table lock method. As used in the cockroach driver. We expose the feature behind a config flag and default the config to advisory lock.

[coveralls]

coverage: 59.183%. first build
when pulling 091ad5d on fortnox-andreas:master
into 691f687 on golang-migrate:master.

[dhui]

@tsfcode Thanks for the PR and for explaining why it's needed!

Did you have plans to also make this change to the postgres driver? (You don't need to)

[tsfcode]

@tsfcode Thanks for the PR and for explaining why it's needed!

Did you have plans to also make this change to the postgres driver? (You don't need to)

I don't have any plans to add it to the postgres driver, since we only use the pgx driver at the moment.

[dhui]

@tsfcode Thanks for addressing the PR feedback!