config reload should re-use unmodified servers

[grobian]

This is the real reason for issues #156 and #157, plus it will fix issue #150.

Now router is a proper object, it can store the servers separately and create uniques as it goes. When a new router object is created it should be able to merge/compare it with the previous one, to take over unmodified servers. In addition, the changes should be reported such that config differences can be spotted.

[grobian]

Reason for this is that currently broken servers at the time of config reload will cause all pending metrics to be dropped, while this in fact isn't necessary if the same server remains in the config.

[grobian]

this is implemented in master