Fixes https://github.com/spring-projects/spring-integration/issues/3872
Adds an implementation for a Postres-compatible notification listener for a `JdbcChannelMessageStore`.
* Introduce `PostgresChannelMessageTableSubscriber.Subscription` contract
* Implement a `PostgresSubscribableChannel`
* Add Javadoc and fix code formatting issues.
* Handle temporary connection loss.
* Add tests for `PostgresChannelMessageTableSubscriber`
* Replace NOTIFY command with pg_notify function call.
* Code style clean up
* Fix some Javadocs
* Use `DataSourceInitializer` in the `PostgresChannelMessageTableSubscriberTests` to populate scripts
* Use Java text block for DB scripts
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