connection_pool: race condition between get_connection cancelation and idle notifications

[anarthal]

Can prevent async_get_connection from completing when an idle connection exists. Low probability.

We have two pending requests A (oldest) and B:

• Connection X goes idle. cancel_one() wakes A only; X is pushed to the idle list.
• A gets cancelled (per-operation cancellation). A wakes from the wait and exits without consuming X.
• X sits in idle_list, but B was never notified. B keeps waiting even though a connection is available.

It won't affect subsequent get_connection requests - these would consume X normally.