Allow self-host users to enable connection pooling for plus datasources

[PClmnt]

Feature Request We can't enable this in cloud due to performance issues, but we should allow self-hosting users to enable connection pooling for plus datasources.

COS

• Environment variable for this setting
• Turn on connection pooling based on the setting

[aderici]

Also needed for Azure appservice users because outbound ports are limited and frequently opening and closing connections cause errors when there's a lot of demand. (many webapps in the same infrastructure sharing the same outbound ip, trying to connect to same azure mysql service inboing ip, there's only 65535 ports available)

Link to azure doc that suggests using connection pooling : https://docs.microsoft.com/en-us/azure/app-service/troubleshoot-intermittent-outbound-connection-errors

[stale[bot]]

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