lwip: support for memory pools

lwip: support for memory pools

Add support for memory pools.

There are now two allocation modes:

(1) heap only, malloc/free are used everywhere (even for memp allocations)
(2) memory pools with custom pools.

The pools are sized to provide good performance for Redis: 1000x256,
1000x512, 1000x1256. Overall this is equivalent to ~2MB, which might be
too much in some cases. In any case, these should be manually tweaked
for performance critical applications.

Increase the defaults for the maximum number of TCP sockets and listeners
(was previously 5, now 64), and expose them via the menuconfig.

Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@neclab.eu>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>