Allow Broadcast channels to block senders when a receiver is not consuming

[sahas-subramanian-frequenz]

What's needed?

When a broadcast receiver is not getting consumed from fast enough and its buffer has filled up, as new messages arrive, older messages in the receiver's buffer start to get dropped.

In some cases, we might not want messages to get discarded, and instead want the sender to wait for the receiver to drain before it can send further messages to the channel.

Proposed solution

Add an optional block_on_full boolean parameter to the get_sender method of the Broadcast channel, which defaults to False.

When true, calls to the send async method will not complete until the value has been sent to all the channels.

Use cases

No response

Alternatives and workarounds

No response

Additional context

No response

[llucax]

This is related to #76, for sends that we know they can't block (because a buffer is used to store it and the message will be dropped if the buffer is full), there is no need to make it async, which has a performance impact. We could make async only sends that can block.