fix: Resolve Clippy CI failures from elided lifetime warnings

The latest Rust nightly compiler (2025-08-27) introduced the
elided-named-lifetimes lint which causes Clippy CI checks to fail
when an elided lifetime ('_) resolves to a named lifetime that's
already in scope.

This commit fixes the Clippy warnings by:
- Making lifetime relationships explicit where 'a is already in scope
- Keeping elided lifetimes ('_) in functions without explicit
  lifetime parameters
- Ensuring proper lifetime handling in the database pool module

Affected files (17 total):
- Database map modules: Handle, Key, and KeyVal references in get,
  qry, keys, and stream operations
- Database pool module: into_recv_seek function

This change resolves the CI build failures without changing any
functionality, ensuring the codebase remains compatible with the
latest nightly Clippy checks.

src/database/map/rev_keys.rs

@@ -10,7 +10,7 @@ use super::rev_stream::is_cached;
 use crate::{keyval, keyval::Key, stream};
 
 #[implement(super::Map)]
-pub fn rev_keys<'a, K>(self: &'a Arc<Self>) -> impl Stream<Item = Result<Key<'_, K>>> + Send
+pub fn rev_keys<'a, K>(self: &'a Arc<Self>) -> impl Stream<Item = Result<Key<'a, K>>> + Send
 where
 	K: Deserialize<'a> + Send,
 {