refactor: remove unnecessary unsafe (#5802)

unsafe impls for `Send` and `Sync` should not be added by default. in
the case of `SlotGuard` removing them does not cause any issues, as the
compiler automatically derives those.

This PR adds requirement to document the unsafety (see
[clippy::undocumented_unsafe_blocks]) and opportunistically adds
`#![deny(unsafe_code)]` to most places where we don't have unsafe code
right now.

TRPL on Send and Sync:
https://doc.rust-lang.org/book/ch16-04-extensible-concurrency-sync-and-send.html

[clippy::undocumented_unsafe_blocks]:
https://rust-lang.github.io/rust-clippy/master/#/undocumented_unsafe_blocks

libs/postgres_backend/src/lib.rs

@@ -2,6 +2,8 @@
 //! To use, create PostgresBackend and run() it, passing the Handler
 //! implementation determining how to process the queries. Currently its API
 //! is rather narrow, but we can extend it once required.
+#![deny(unsafe_code)]
+#![deny(clippy::undocumented_unsafe_blocks)]
 use anyhow::Context;
 use bytes::Bytes;
 use futures::pin_mut;