Add support for io_uring queues cancellation

[danielealbano]

To properly implement the database snapshotting, the pending I/O operation need to be cancelled without terminating the ring itself as this is needed to carry out the I/O operations to generate the database snapshot.

This will require introducing an ad hoc flow in the events processing loop that, upon the termination request, will:

• close all the pending connections
• set a next run in the past
• wait until the next run field changes before terminating

A new config flag has to be added to control this behaviour as the second and third step must be carried out only if a snapshot on shutdown is requested.

Potentially on more recent kernels it's possible to issue a cancellation request with the flag IORING_ASYNC_CANCEL_ANY

More info at https://man7.org/linux/man-pages/man3/io_uring_prep_cancel64.3.html

[danielealbano]

This functionality has been implemented in v0.4 using an alternative approach therefore it's not necessary anymore.