client/finality-grandpa: Reintegrate periodic neighbor packet worker (#4631)

The `NeighborPacketWorker` within `client/finality-grandpa` does two
things:

1. It receives neighbor packets from components within
`client/finality-grandpa`, sends them down to the `GossipEngine` in
order for neighboring nodes to receive.

2. It periodically sends out the most recent neighbor packet to the
`GossipEngine`.

In order to send out packets it had a clone to a `GossipEgine` within
an atomic reference counter and a mutex. The `NeighborPacketWorker` was
then spawned onto its own asynchronous task.

Instead of running in its own task, this patch reintegrates the
`NeighborPacketWorker` into the main `client/finality-grandpa` task not
requiring the `NeighborPacketWorker` to own a clone of the
`GossipEngine`.

The greater picture

This is a tiny change within a greater refactoring. The overall goal is
to **simplify** how finality-grandpa interacts with the network and to
**reduce** the amount of **unbounded channels** within the logic.

Why no unbounded channels: Bounding channels is needed for backpressure
and proper scheduling. With unbounded channels there is no way of
telling the producer side to slow down for the consumer side to catch
up.  Rephrased, there is no way for the scheduler to know when to favour
the consumer task over the producer task on a crowded channel and the
other way round for an empty channel.

Reducing the amount of shared ownership simplifies the logic and enables
one to use async-await syntax-suggar, given that one does not need to
hold a lock across poll invocations. Using async-await enables one to
use bounded channels without complex logic.