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_ffi/src/lib.rs

@@ -8,6 +8,7 @@
 // modules included with the postgres_ffi macro depend on the types of the specific version's
 // types, and trigger a too eager lint.
 #![allow(clippy::duplicate_mod)]
+#![deny(clippy::undocumented_unsafe_blocks)]
 
 use bytes::Bytes;
 use utils::bin_ser::SerializeError;
@@ -20,6 +21,7 @@ macro_rules! postgres_ffi {
             pub mod bindings {
                 // bindgen generates bindings for a lot of stuff we don't need
                 #![allow(dead_code)]
+                #![allow(clippy::undocumented_unsafe_blocks)]
 
                 use serde::{Deserialize, Serialize};
                 include!(concat!(