[BUG] AsyncCurrentValueSubject may miss values when a consumer is added concurrently with send

[ERussel]

Current Behavior

AsyncCurrentValueSubject may miss a value when a new consumer is added
concurrently with send.

In handleNewConsumer(), the current value is read from the state
before the new channel is registered:

let (terminalState, current) = state.withCriticalRegion {
    ($0.terminalState, $0.current)
}

...

asyncBufferedChannel.send(current)

...

state.withCriticalRegion {
    $0.channels[consumerId] = asyncBufferedChannel
}

Since reading current and registering the channel are separate
operations, send can interleave between them.

For example, assuming the current value is A:

Consumer                         Producer

read current = A
unlock

                                 send(B)
                                   current = B
                                   // new consumer isn't registered

channel.send(A)
register channel

The consumer receives A, but never receives B, even though B was
sent after the consumer started subscribing.

Unless another value is sent, the consumer is now observing a stale
value while the subject's current value is B.

There appears to be a similar race with termination: the subject can
terminate after terminalState is checked but before the new channel is
registered, potentially leaving the newly registered consumer unaware
that the subject has already terminated.

Expected Behavior

Establishing a new subscription should be linearizable with respect to
send and termination.

For a concurrent subscription and send(B), either:

subscribe -> send(B)

consumer receives: A, B

or:

send(B) -> subscribe

consumer receives: B

should be possible, but the newly subscribed consumer should not
permanently miss B.