[SPDK] nvmf: Async event support for discovery log change

Hi SPDK community,

I have submitted a patch for adding async event support for discovery log
change in nvmf targets.
4870: nvmf: Async event support for discovery log change |
https://review.spdk.io/gerrit/c/spdk/spdk/+/4870 .

According to nvme fabrics spec 1.1, if the connect command specifies a
non-zero keep alive timer value and the discovery controller does not
support asynchronous events then we need to return Connect Invalid. Since
SPDK currently does not implement this check I have instead added support
for asynchronous events for discovery log changes. This allows a host with
persistent connection to the discovery subsystem to automatically be
notified if any new subsystems are available.

Please check the patch when possible and let me know if this is the right
approach.. Also let me know if any issues need to be fixed.

Thanks.

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