[MEDIUM] introduce pipe pools

[MEDIUM] introduce pipe pools

A new data type has been added : pipes. Some pre-allocated empty pipes
are maintained in a pool for users such as splice which use them a lot
for very short times.

Pipes are allocated using get_pipe() and released using put_pipe().
Pipes which are released with pending data are immediately killed.
The struct pipe is small (16 to 20 bytes) and may even be further
reduced by unifying ->data and ->next.

It would be nice to have a dedicated cleanup task which would watch
for the pipes usage and destroy a few of them from time to time.