Run TCP and UDP scans consecutively, not concurrently

**What is the problem your feature is trying to solve?**
As evidenced in #885 and #1290, constrained devices have been seen to struggle as it is with Testrun's insane (nmap's [terminology](https://nmap.org/book/performance-timing-templates.html), not mine!) [T5 timing approach](https://github.com/google/testrun/blob/b476cde69d6af36b2b52245454f33e90cb4b23e5/modules/test/services/python/src/services_module.py#L216).

This isn't helped by the fact the UDP scan runs concurrently, and so the device is also getting a dose of UDP port scans at the same time as it is getting a dose of TCP scans.

**Describe the solution you think would solve the problem**
My personal opinion is that these scans should run consecutively, not concurrently, to avoid overloading the DUT.