MINOR: quic: define sbuf pool

MINOR: quic: define sbuf pool

Define a new buffer pool reserved to allocate smaller memory area. For
the moment, its usage will be restricted to QUIC, as such it is declared
in quic_stream module.

Add a new config option "tune.bufsize.small" to specify the size of the
allocated objects. A special check ensures that it is not greater than
the default bufsize to avoid unexpected effects.