dhcpv4: lazy store statefiles

dhcpv4: lazy store statefiles

Currently, the dhcpv4 server saves the statefile on every change and
also calls the leasetrigger script every time the statefile is updated
(which triggers a dnsmasq reload).

In addition, odhcpd wakes up every second (see dhcpv4_valid_until_cb())
to go through all existing leases to see if any lease has expired.

With this change, the wakeups are reduced to every 5 seconds, and the
statefile is only written (if necessary) during that wakeup.

Before this change (without any leasetrigger, test on my laptop, not a
real OpenWrt device):

$ time sudo ./build/dhcpdig -4 benchmark foo-client

Thread[**]: req 10000101/0 rel 10000101/0 rep 10000101/0

real	55m29.406s
user	0m0.005s
sys	0m0.010s

(This is a simple benchmark tool I wrote, it runs 10 threads which
divide the DHVPv4 pool into 10 chunks and then performs 1 million random
addr req/release per thread in a loop).

After this change:

$ time sudo ./build/dhcpdig -4 benchmark foo-client

Thread[**]: req 10000101/0 rel 10000101/0 rep 10000101/0

real	1m48.123s
user	0m0.005s
sys	0m0.005s

A 3082% speedup.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/298
Signed-off-by: Álvaro Fernández Rojas <[email protected]>