ntp: add support for multiple suspended sockets

ntp: add support for multiple suspended sockets

With some hardware it takes milliseconds to get the HW TX timestamp.

Rework the code to handle multiple suspended client-only sockets at the
same time in order to allow longer timeouts, which may overlap for
different sources. Instead of waiting for the first read event simply
suspend the socket and create timeout when the HW TX timestamp is
requested.