Fix last contacted

[dsiganos]

Fix unit test network.last_contacted (#3285)

The unit test network.last_contacted has the following problems:
* regular keepalive messages sent automatically can interfere with the test
* the test should wait for the clock to move at least one tick to eliminate the possibility of the clock not having the time to move forward
* the test waits for a the keepalive counter to be incremented and then expects the last_packet_received timestamp to be updated but the timestamp is updated after the counter is incremented, which is a race condition

The solution:
* send a second keepalive to handle the race condition between setting the timestamp and incrementing the counter
* send a third keepalive to handle the case where an automatic keepalive message is already in flight
* wait for the clock to move after reading it and before continuing

Assumptions:
We assume that there will be no more than 1 keelalive in flight.
Of course, in theory, there could be multiple keepalives queued up but
for the purposes of this test, we assume max 1 keepalive in flight.
It is not an unreasonable assumption in this test case.