Port the `stm-containers` based `DiagnosticStore` from HLS

[michaelpj]

HLS has its own implementation of DiagnosticStore that uses the STM map from stm-containers. This is a good idea, not sure why we didn't just upstream it at the time.

https://github.com/haskell/haskell-language-server/blob/b91c9076367123e983087ed305d183288a23f494/ghcide/src/Development/IDE/Core/Shake.hs#L1306