Remove client close handle
Remove client close handle

Remove the client's close handle. This was eclipsed by the
`client::bridge::gateway::ShardManager`, which is a public interface
giving full control over connected shards owned by the instance of the
client (as opposed to the purpose of the handle which was a simple
"shutdown" signal).

Additionally, more documentation has been added to
`Client::shard_manager`, now including a sample scenario of how to
shutdown the bot after some amount of time has passed.

Upgrade path:

Refer to the documentation for `Client::shard_manager` on a sample
scenario on how to switch from the close handle to the ShardManager.

zeyla/serenity,acdenisSK/serenity,Flat/serenity,Lakelezz/serenity,zeyla/serenity.rs