Refactor: Fix: impl XX for associated type conflict in another crate

This commit addressed an issue of implementing trait for associated
types, by replacing implementation for associated type with concrete
type `pb::Vote`.

- Related issues: rust-lang/rust#51445

- The minimized reproduce repo is:
  https://github.com/drmingdrmer/conflict-trait-in-main

Explanation:

If one crate `lib.rs` implements a trait(`Default`) for an associated
type(`<() as Container>::Item`, which is `Foo`), in another crate
`main.rs`, it seems the compiler treats the associated type(`<() as
Container>::Item`) as a generic type `T` and assumes `T` would
be any type and results in the conflict with a local type that tries to
implement `Default`, while actually `Foo` and `Wow` are actually
different types.

cargo repo:

```
▾ src/
    lib.rs
    main.rs
```

`lib.rs`:

```rust
pub trait Container {
    type Item;
}

impl Container for () {
    type Item = Foo;
}

pub struct Foo;

impl Default for <() as Container>::Item {
    fn default() -> Self { Foo }
}
```

`main.rs`:

```rust
// Make main.rs depends on lib.rs
use t_conflict::Container as _;

struct Wow;
impl Default for Wow {
    fn default() -> Self { Wow }
}

fn main() {}
```

`cargo build` yield this error:
```
error[E0119]: conflicting implementations of trait `Default` for type `Wow`
 --> t-conflict/src/main.rs:7:1
  |
7 | impl Default for Wow {
  | ^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `t_conflict`:
          - impl Default for <() as Container>::Item;
```

examples/raft-kv-memstore-grpc/src/pb_impl/impl_vote.rs

@@ -2,12 +2,13 @@ use std::fmt;
 
 use openraft::vote::RaftVote;
 
-use crate::typ::*;
+use crate::pb;
+use crate::typ::LeaderId;
 use crate::TypeConfig;
 
-impl RaftVote<TypeConfig> for Vote {
+impl RaftVote<TypeConfig> for pb::Vote {
     fn from_leader_id(leader_id: LeaderId, committed: bool) -> Self {
-        Vote {
+        pb::Vote {
             leader_id: Some(leader_id),
             committed,
         }
@@ -22,7 +23,7 @@ impl RaftVote<TypeConfig> for Vote {
     }
 }
 
-impl fmt::Display for Vote {
+impl fmt::Display for pb::Vote {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
             f,