Read packets directly from the read ahead cache when completely cached

Every packet is read by ma_real_read() with two generic ma_pvio_cache_read() calls (header, then payload), even when the whole packet is already in the read ahead cache, which is the usual case for the rows of a result set.

New ma_net_read_cached() handles that case in ma_net_read(): it decodes the header in place and copies the payload once. Packets that are not completely cached, multi packets and the compressed protocol use the existing code.