fix: `Client#end` callback being called multiple times when first is no-op (#3676)

* test: unintended listener after no-op `Client#end` callback

* fix: `Client#end` callback being called multiple times when first is no-op

(and unwanted retained listener even when not called multiple times)

* fix: Prevent multiple callbacks in pg/native `Client#end`, and align pre-connect behaviour closer to pg

As usual, the native client is extra full of bugs and inconsistencies, so this is just “good enough”.