Review bucket_gocb concurrent op limits

[adamcfraser]

We're currently capping the number of concurrent operations Sync Gateway pushes into the gocb pipeline, to try to avoid the overhead of queue overflow/retry processing.

However, the current cap doesn't account for the relationship between gocb queue capacity and the number of server nodes - gocb supports 2048 queued operations per node, not overall.

Need to adjust the targeted concurrent ops based on cluster size.

This needs to consider the case where the number of server nodes changes post-bucket instantiation.