Add interface for long running queries

[kevinwcyu]

Adding interface for plugins to implement for asynchronous queries.

There's some changes to the variables MissingMultipleConnectionsConfig and MissingDBConnection. I'll open a separate PR for those changes. Only have them in here to fix this warning ... should have name of the form ErrFoo (ST1012) go-staticcheck while working through this PR.

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[kevinwcyu]

Closing for #69