fix every clippy warning possible, remove io_uring as default feature

this project's codebase is so horrendous, im shocked that no one has ran
clippy at all. it had ~200 total lint warnings, some with performance
issues and unsoundness, and the rest just very ugly codebase. i have sat
down and fixed as many of these as possible and i am exhausted.
i haven't fixed some extremely complex ones, but i brought it down from
~200 to ~30.

i have also removed io_uring as a default feature due to it falling
under the same category as linux eBPF: major kernel attack surface for
minimal performance gains. this also makes it impossible to cross-compile
from macOS to Linux because io_uring does not exist in Darwin land.
there are far better ways to achieve better performance than io_uring on
the codebase level.

Signed-off-by: strawberry <june@girlboss.ceo>

src/service/sending/mod.rs

@@ -132,7 +132,7 @@ impl Service {
         for (key, outgoing_kind, event) in self.db.active_requests().filter_map(|r| r.ok()) {
             let entry = initial_transactions
                 .entry(outgoing_kind.clone())
-                .or_insert_with(Vec::new);
+                .or_default();
 
             if entry.len() > 30 {
                 warn!(