[2.8.3 Backport] Limit the number of open N1QL connections

Sync Gateway limits the number of concurrent view queries, to avoid too many open HTTP requests between SG and the view endpoint, which can result in SG running out of available sockets.

The same needs to be done for N1QL queries, as the go SDK is also issuing a separate HTTP request for each query.

Since SG won't be issuing view and N1QL requests against the same bucket, the existing waitForAvailableViewOp/releaseViewOp can be reused for N1QL queries.