log: free resources while event loop is running

The 'log' module uses fibers internally for logs rotation sake and
before we can free log's resources (on program exit) we need to wait
until rotation is complete, which implies that events loop is still
running. But we break the event loop in `on_shutdown_f` trigger and
calling any events based functionality later cause unexpected results
because fibers are no loner valid to use. Thus move `say_logger_free`
call into `on_shutdown_f` body where fibers are still alive.

N.B. Testing the issue is sensitive to timings, during local tests
found that minimal delay 1ms is enough to trigger, thus ERRINJ_LOG_ROTATE
get increased.

Fixes #4450

NO_DOC=bugfix

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>