refactor check handling functions

there are now 2 spots ([1](https://github.com/grafeas/voucher/blob/main/server/check.go#L17), [2](https://github.com/grafeas/voucher/blob/main/subscriber/check.go#L14)) where we have similar enough logic and we should consider refactoring these into a single spot. things to keep in mind:

- we probably want to split this up into multiple functions
- we want to avoid import loops (because we're using the `config` pkg to initialize new clients and whatnot)

Start by comparing the duplicated logic in server/check.go and subscriber/check.go. Review how the config package initializes clients and trace the package dependencies before deciding where shared functions can live. Done means the check handling is consolidated without introducing import loops.

Written by the indexing model from the issue text.