MINOR: traces: always mark trace_source as thread-aligned

MINOR: traces: always mark trace_source as thread-aligned

Some perf profiles occasionally show that reading the trace source's
state can take some time, which is not expected at all. It just happens
that the trace_source is not cache-aligned so depending on linkage, it
may share a cache line with a more active variable, thereby inducing a
slow down to all threads trying to read the variable.

Let's always mark it aligned to avoid this. For now the problem was not
observed again.