Set a max waiting clients in metadata backend.

[dgaubert]

Do not hang request forever when metadata backend is not able to deliver requests. It's preferable, response with an error before pushing the process to its limits. A symptom of these issues is when Tiler drops a bunch of log entries as follows:

[2019-05-23 10:16:18.235] [INFO] console - {"name":"windshaft-server","db":8,"action":"acquire","elapsed":45,"waiting":15}

The more entries like this in logs, the worse is performing metadata backend. It makes the customer's request remaining hung in Tiler, memory grows, and GC enters for a long time turning the process unresponsive, and monit triggers a restart of the service, dropping incoming requests.