Remove async_trait dependency in deadpool_postgres

[bikeshedder]

The removal of async_trait in deadpool-postgres 0.13.0 caused some code to break:

• Code that compiles under deadpool-postgres 0.12.1 fails to compile under 0.13.2 #323

The tests now include the minimum reproduction example of #323 so future changes of this code don't break this by accident.

[CathalMullan]

The inference issue was fixed by rust-lang/rust#144064, so once the MSRV is at 1.92 or later, removing async_trait should be safe.