Add phosphor tracing for missing Ephemeral-related tasks

We do not have phosphor trace events for some of the ephemeral related nonIO tasks - e.g. EphTombstoneStaleItemDeleter and EphTombstoneHTCleaner.

We should look at adding appropriate tracing events (see tasks.cc; TRACE_EVENT macros) to the nonIO tasks which are not currently traced; for us to better understand their behaviour.