MINOR: quic: define new functions for frame alloc

MINOR: quic: define new functions for frame alloc

Define two utility functions for quic_frame allocation :
* qc_frm_alloc() is used to allocate a new frame
* qc_frm_dup() is used to allocate a new frame by duplicating an
  existing one

Theses functions are useful to centralize quic_frame initialization.
Note that pool_zalloc() is replaced by a proper pool_alloc() + explicit
initialization code.

This commit will simplify implementation of the per frame retransmission
limitation. Indeed, a new counter will be added in quic_frame structure
which must be initialized to 0.

This should be backported up to 2.7.