[Bro] write_expire computational time

[Jan Grashöfer]

> Oh! good point. That also reminds me that if your expire times are < 10seconds then you'd have to redef table_expire_interval to a value < 10 seconds.

I was just typing the same! You can also influence the delay and step
size using table_expire_interval and table_incremental_step
(https://www.bro.org/sphinx/scripts/base/init-bare.bro.html?#id-table_expire_delay).

Jan