MEDIUM: dynbuf: Add a pool for large buffers with a configurable size

MEDIUM: dynbuf: Add a pool for large buffers with a configurable size

Add the support for large bufers. A dedicated memory pool is added. The size
of these buffers must be explicitly configured by setting
"tune.bufsize.large" directive. If it is not set, the pool is not
created. In addition, if the size for large buffers is the same than for
regular buffer, the feature is automatically disable.

For now, large buffers remain unused.