Make transactions undroppable

coordinator/cosign/src/delay.rs literally demonstrates how we'd need to rewrite
our handling of transactions with this change. It can be cleaned up a bit but
already identifies ergonomic issues. It also doesn't model passing an &mut txn
to an async function, which would also require using the droppable wrapper
struct.

To locally see this build, run

RUSTFLAGS="-Zpanic_abort_tests -C panic=abort" cargo +nightly build -p serai-cosign --all-targets

To locally see this fail to build, run

cargo build -p serai-cosign --all-targets

While it doesn't say which line causes it fail to build, the only distinction
is panic=unwind.

For more context, please see #578.

common/db/src/parity_db.rs

@@ -37,7 +37,7 @@ impl Get for Arc<ParityDb> {
 }
 impl Db for Arc<ParityDb> {
   type Transaction<'a> = Transaction<'a>;
-  fn txn(&mut self) -> Self::Transaction<'_> {
+  fn unsafe_txn(&mut self) -> Self::Transaction<'_> {
     Transaction(self, vec![])
   }
 }