MINOR: mux-h2: assign a limited frames processing budget

MINOR: mux-h2: assign a limited frames processing budget

This introduces 3 new settings: tune.h2.be.max-frames-at-once and
tune.h2.fe.max-frames-at-once, which limit the number of frames that
will be processed at once for backend and frontend side respectively,
and tune.h2.fe.max-rst-at-once which limits the number of RST_STREAM
frames processed at once on the frontend.

We can now yield when reading too many frames at once, which allows to
limit the latency caused by processing too many frames in large buffers.
However if we stop due to the RST budget being depleted, it's most likely
the sign of a protocol abuse, so we make the tasklet go to BULK since
the goal is to punish it.

By limiting the number of RST per loop to 1, the SSL response time drops
from 95ms to 1.6ms during an H2 RST flood attack, and the maximum SSL
connection rate drops from 35.5k to 28.0k instead of 11.8k. A moderate
SSL load that shows 1ms response time and 23kcps increases to 2ms with
15kcps versus 95ms and 800cps before. The average loop time goes down
from 270-280us to 160us, while still doubling the attack absorption
rate with the same CPU capacity.

This patch may usefully be backported to 3.3 and 3.2. Note that to be
effective, this relies on the following patches:

  MEDIUM: sched: do not run a same task multiple times in series
  MINOR: sched: do not requeue a tasklet into the current queue
  MINOR: sched: do not punish self-waking tasklets anymore
  MEDIUM: sched: do not punish self-waking tasklets if TASK_WOKEN_ANY
  MEDIUM: sched: change scheduler budgets to lower TL_BULK