Record timer details and enable validation tests for record&replay

[smarr]

The new uniform record&replay (#337) does not yet handle external events, including the timer.

We essentially need to reimplement the timer using the new tracing infrastructure in TracingActivityThread.

This will allow us to reenable the validation tests here: https://github.com/smarr/SOMns/pull/337/files#diff-f1dab3cc0c0b97293c62c8db275d5793R118