Use an async-aware Mutex implementation

[imobachgs]

Clippy reports that in our code we call await while holding a lock. Example:

warning: this `MutexGuard` is held across an `await` point
   --> agama-dbus-server/src/network/dbus/tree.rs:145:13
    |
145 |         let mut objects = self.objects.lock();
    |             ^^^^^^^^^^^
    |
    = help: consider using an async-aware `Mutex` type or ensuring the `MutexGuard` is dropped before calling await
note: these are all the `await` points this lock is held through
   --> agama-dbus-server/src/network/dbus/tree.rs:145:9
    |
145 | /         let mut objects = self.objects.lock();
146 | |         for path in objects.connections.values() {
147 | |             self.remove_connection_on(path.as_str()).await?;
148 | |         }
149 | |         objects.connections.clear();
150 | |         Ok(())
151 | |     }
    | |_____^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_lock

There are two possible solutions:

• Make sure than the Mutex is drop before calling await (see [RFC] Avoid calling await while holding a lock #663).
• Use an async-aware Mutex implementation (this).

This PR replaces parking_lot Mutex with the one included in the futures. Both of them are already included as dependencies of other crates.

Follow-up: #703.

[coveralls]

coverage: 72.385% (+0.002%) from 72.383% when pulling 0dbb5f9 on imobachgs:async-aware-mutex into 2e11beb on openSUSE:master.