router: add callro methods based on node zone

[eugenepaniot]

Pick a replica according prefered zone (highest priority replica zone) in round-robin manner.

Current callr* calls use only first replica. Current callbr* calls use replicas in round-robin manner only, without zonal preferences that leads to extra cross zonal traffic (that costs some pennies) and higher latency.

Current PR suggests two new methods and new load-balancing (replica selection) algorithm:

• callbzro - Similar to existing callbro - prefer, but not limit, Inner zone balanced method with round-robin;
• callbzre - Similar to existing callbre - but prefer, but not limit, Inner zone balanced method with round-robin;

The algorithm is quite simple but useful, it resets round-robin cursor balance_i into the main position (1 - highest priority replica) when reches the last replica in the same zone. It is possible as all replicas already sorted in the priority_list by weight(actually relies on replica zone and zone_weights).

[Serpentian]

Thanks for the patch! Looks great! Please get acquainted with code review procedure and make sure your PR comply with our styling rules. For example:

• A patch should include tests for the new features;
• The title of the commit should be router: add callro methods based on node zone. The commit message should explain why and what you did;
• We don't usually use comments after a line of code.

[eugenepaniot]

Sorry, very busy now to implement tests. Will try to do it soon. It will be very helpful if someone from QA Team helps with it. Thank you.