The server weights were ignored for the first connection and servers with a
weight of 0 would get connections if the connection count was high enough.
This fixes the weighting behavior so that when connections are created,
servers with a weight of 0 will be ignored as long as there is a server
with a positive weight available.
With this change, three servers configured with weights 2, 1 and 0 would
get connections balanced in the following way:
weight = 2, 66% of connections
weight = 1, 33% of connections
weight = 0, 0% of connections
If the server with the weight of 2 would go down, the server with the weight
of 1 would get 100% of the connections. If both servers with positive weights
go down, the server with the weight of 0 would be used.
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