tw_timer_expire_timers() - add a maximum to the number of expiration per call

tw_timer_expire_timers() - add a maximum to the number of expiration per call

The idea is to prevent a huge processing burst if, say, the network goes
down 10' for some reason, and so that we don't need to expire 1M timer
sessions on the first call.

The maximum is not an exact value, but a value after which the
expiration process is postponed until the next call.
That way, we don't have to process the same tick twice, nor to unlink
timers once at a time when processing a tick.
The fact that a timer slot could contain many entries should be dealt
with by changing the number of ticks per second.

Change-Id: I892d07f965094102a3d53e7dbf4e6f5ad22d4967
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>