Only select chunks from submissions that match the given key-value set

[jerbaroo]

As the AI team we are also interested in a new strategy: SelectOnly

From https://github.com/channable/opsqueue/blob/master/doc/proposals/strategies_and_metadata.md:

/// Only select chunks from submissions that match the given key-value set when submission
SelectOnly(meta_field: String, value: MetaValue, underlying: Strategy)

For us it would be a key ingredient in implementing blue/green deployments for our AI model. The meta_field would then be the model version.

Originally posted by @SemMulder in #95

[jerbaroo]

@SemMulder I like the idea and have moved it to its own issue.

Currently all the strategies are ways of choosing an ordering, but the suggestion seems more like a filter. So perhaps it should be an optional parameter to pass alongside a strategy