MINOR: dynbuf/mux-h1: use different criticalities for buffer allocations

MINOR: dynbuf/mux-h1: use different criticalities for buffer allocations

While it could certainly still be improved, this first approach consists
in assigning buffers like this in the H1 mux:
  - h1c->obuf : DB_MUX_TX
  - h1c->ibuf : DB_MUX_RX
  - h1s->rxbuf: DB_SE_RX

That's done via 3 distinct functions for better code clarity, and it
also allowed to move the missing buffer flags assignment there.

Among possible improvements would be to take into consideration the
state of the parser (i.e. no data yet vs data, or headers vs payload)
so that even server beginning of response or pure payload can be lowered
in priority.