[OPTIM] task: reduce the number of calls to task_queue()

[OPTIM] task: reduce the number of calls to task_queue()

Most of the time, task_queue() will immediately return. By extracting
the preliminary checks and putting them in an inline function, we can
significantly reduce the number of calls to the function itself, and
most of the tests can be optimized away due to the caller's context.

Another minor improvement in process_runnable_tasks() consisted in
taking benefit from the processor's branch prediction unit by making
a special case of the process_session() callback which is by far the
most common one.

All this improved performance by about 1%, mainly during the call
from process_runnable_tasks().