Fix: per-uid buffers should only be flushed once on stop

They were flushed once per application previously, causing workload with many
applications to trigger too many flush on stop, which in turn caused buffer
padding bloat.

* The pseudo-code for stop behavior on per-uid buffers was:

flush session's buffers
for each application:
  - stop tracing
  - wait quiescent
  - flush buffers
  - push metadata

Now doing, for per-uid buffers:

for each application:
  - stop tracing
  - wait quiescent
  - push metadata
flush session's buffers

* And for per-pid buffers:

Previously:

for each application:
  - stop tracing
  - wait quiescent
  - flush buffers
  - push metadata

Now:

for each application:
  - stop tracing
  - wait quiescent
  - push metadata
for each application:
  - flush buffers

Fixes #497

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>