sync: remove broadcast channel slot level closed flag

[hds]

Fixes #4814

Motivation

There is an off by one error discovered by @nathaniel-daniel in #4814 which caused a
receiver which is created after a channel has already been closed to receive Empty
instead of Closed when calling try_recv. This would also cause the async function
recv to wait forever.

The broadcast channel stores two closed flags, one in each slot in the buffer (to indicate
the slot after the last filled slot) and a channel wide flag on the tail member. The
implementation misses the slot level closed flag because its next field doesn't account
for the increment when it was added.

As @nathaniel-daniel mentions in their issue, this can be solved by updating the next
field on the new receiver (seriously, this observation helped me a lot, thank you!).

However, after digging into the internals of the broadcast channel a bit, it looks like we
don't need both the slot level and the channel level closed flags. Instead, we can rely
on only the channel level closed flag.

Instead, a receiver will continue to receive buffered values until it reaches the tail of the
buffer. At that point we check whether the channel level closed flag is set and return
Empty or Closed. This fixes the off by one error and logically should work in all cases
(and the tests all pass without modification).

Solution

The broadcast channel allows multiple senders to send messages to
multiple receivers, where each receiver receives messages starting from
when it subscribes. After all senders are dropped, the receivers will
continue to receive all waiting messages in the buffer and then receive
a Closed error.

To mark that a channel has closed, it stores two closed flags, one on
the channel level and another in the buffer slot after the last used
slot (this may also be the earliest entry being kept for lagged
receivers, see #2425).

However, we don't need both closed flags, keeping the channel level
closed flag is sufficient.

Without the slot level closed flag, each receiver receives each message
until it is up to date and for that receiver the channel is empty. Then,
the actual return message is chosen depending on the channel level
closed flag; if the channel is NOT closed, then Empty is returned, if
the channel is closed then Closed is returned instead.

With the modified logic, there is no longer a need to append a closed
token to the internal buffer (by setting the slot level closed flag on
the next slot). This fixes the off by one error described in #4814,
which caused a receiver which was created after the channel was already
closed to get Empty from try_recv (or hang forever when calling
recv) instead of receiving Closed.

As a bonus, we save a single bool on each buffer slot.

Refs: #4814

[Darksonn]

This looks good.