Motivation:
The behaviour of the FixedChannelPool.release was inconsistent with the
SimpleChannelPool implementation, in that given promise is returned.
In the FixedChannelPool implementation a new promise was return and
this meant that the completion of that promise can be different.
Specifically on releasing a channel to a closed pool, the parameter
promise is failed with an IllegalStateException but the returned one
will have been successful (as it was completed by call to super
.release)
Modification:
Return the given promise as the result of FixedChannelPool.release
Result:
Returned promise will reflect the result of the release operation.
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