Add optional round-robin channel selection for affinity binding calls.

[nimf]

Currently a binding call (as any other call without an affinity key) gets the least busy channel, i.e. channel that has fewest active streams. The current logic may not suit all the cases for binding new affinity keys in an optimal way.

It works well when the upstream issues bind calls infrequently and uses the affinity keys from these calls immediately. This way the channels used for the bind calls get utilized by the time a next bind call is issued and affinity keys remain balanced across channels.

But if the upstream makes several bind calls one after another and does not use the returned affinity keys right away, then the affinity keys will not be balanced.

For example:

Let's assume we have this situation: we have 3 long running calls on channel 1 and 3 each and 1 call on the channel 2:

Channels	Affinity keys	Active streams
1	5	3
2	5	1
3	5	3

New bind call arrives and gets routed to channel 2 because it has the fewest active streams.
The bind call returns 5 new affinity keys that is bound to channel 2:

Channels	Affinity keys	Active streams
1	5	3
2	10	1
3	5	3

Another bind call arrives and gets another 5 new keys:

Channels	Affinity keys	Active streams
1	5	3
2	15	1
3	5	3

This pattern may be continued to incur more imbalance.

If the upstream always requests the bind calls with a fixed number of affinity keys and never does unbind calls then we can round-robin bind calls to guarantee balanced distribution of affinity keys.

This PR introduces the option to round-robin bind calls.