net: convert SYN cache to use async::serial_timer_task instead of callouts

This fixes lock order inversion problem mentioned in #287 for SYN
cache.

This problem may affect all places which use callout_init_mtx() or
callout_init_rw(). The only place which is left after this change is
in in_lltable_new(), which is creating a timer for expiring ARP
entries.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>