Reworked HTTP/2 release after an exchange is terminated.
Previously, the release was bound to 2 events: onStreamClosed(),
introduced for #2796, and exchangeTerminated().
Unfortunately, if the former happens before the latter and
closes the connection, the latter will see the exchange as
aborted, while in fact it was successful, causing what
reported in #5147, an AsynchronousCloseException.
Now, the release is always performed by the exchangeTerminated()
event. With respect to #2796, the stream is always already
closed by the time the exchangeTerminated() event fires (it
was not before).
Reworked the implementation of RoundRobinConnectionPool using
a lock and aggressively trying to open new connections.
A second fix is related to HttpDestination.release(Connection).
If the connection is closed for e.g. overuse, we need to trigger
the processing of queued requests via send(create: true).
Signed-off-by: Simone Bordet <simone.bordet@gmail.com>
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