Add a metadata service for matching-ws

[djtfmartin]

It would be useful to provide a metadata service for the matching-ws. This service could report:

• The dataset in checklistbank that has been index, and the version
• git version info
• basic statistics on the service (number of taxa at different ranks etc)
• When the index was generated
• Index size on disk

[mdoering]

we should consider to extend the CLB model to hold that information. We do already track stats for specific versions, so these could be reused:

https://api.checklistbank.org/dataset/1010/import?limit=10

See compound datasetKey & attempt key