Directory mirrors should check accounting usage more frequently
After merging legacy/trac#12538 (moved) and legacy/trac#18616 (moved), Tor can wait up to 18 hours before checking whether the accounting usage has increase so much that we want to stop advertising DirPort and begindir support.
This is not great for relays with daily accounting periods.
I suggest we check every hour to see whether the result of router_should_be_directory_server() has changed. If so, we should mark_my_descriptor_dirty() with an appropriate message about deciding to advertise or stop advertising directory service support / DirPort.
This likely also means separating the calculation part of router_should_be_directory_server() from the logging, otherwise we will log duplicate messages each time we change our minds about our accounting.