getIPAddress: fix undefined index 'class'

in case when scanIPSpace found nothing, the result of getIPAddress was
not containing the 'class' key
scanIPSpace: partial scan using flags

It is sometimes needed to find a particular IP address usage:
slb rs, object's allocation, router allocation and so on.
Full scan for all IP usage types could take too long and/or consume
too much memory.

The $flag argument added to the scanIPSpace, scanIPv4Space, scanIPv6Space
functions to filter out SQL queries that are not needed.

findNetRouters: new function to effectively search for a router allocation.
filterOwnAddrList: filtering the own_addrlist is separated from the loadIPAddrList.
use findNetRouters instead of findRouters
new scanIPSpace_hook hook

It is possible to load the used IP address list from third-party data
source, not only from the RT tables.