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Prevent unexpected UDP client read callbacks

Ondřej Surý requested to merge 3545-prevent-unexpected-client-read-callbacks into main

The network manager UDP code was misinterpreting when the libuv called the udp_recv_cb with nrecv == 0 and addr == NULL -> this doesn't really mean that the "stream" has ended, but the libuv indicates that the receive buffer can be freed. This could lead to assertion failure in the code that calls isc_nm_read() from the network manager read callback due to the extra spurious callbacks.

Properly handle the extra callback calls from the libuv in the client read callback, and refactor the UDP isc_nm_read() implementation to be synchronous, so no datagram is lost between the time that we stop the reading from the UDP socket and we restart it again in the asychronous udpread event.

Add a unit test that tests the isc_nm_read() call from the read callback to receive two datagrams.

Closes #3545 (closed)

Edited by Ondřej Surý

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