MINOR: tasks: do not keep cpu and latency times in struct task

MINOR: tasks: do not keep cpu and latency times in struct task

It was a mistake to put these two fields in the struct task. This
was added in 1.9 via commit 9efd7456e ("MEDIUM: tasks: collect per-task
CPU time and latency"). These fields are used solely by streams in
order to report the measurements via the lat_ns* and cpu_ns* sample
fetch functions when task profiling is enabled. For the rest of the
tasks, this is pure CPU waste when profiling is enabled, and memory
waste 100% of the time, as the point where these latencies and usages
are measured is in the profiling array.

Let's move the fields to the stream instead, and have process_stream()
retrieve the relevant info from the thread's context.

The struct task is now back to 120 bytes, i.e. almost two cache lines,
with 32 bit still available.