BVC-47 :: Lack of Diagnosis keys download limit

[obrien-j]

We've set a maxKeysPerFile constant, but never actually use it when preparing the key bundles.

https://github.com/cds-snc/covid-shield-server/blob/e14893d09e065bf50039e9502a48cfce969d9704/pkg/retrieval/retrieval.go#L18

This is likely a non-issue, at least for the initial launch, but will obviously become problematic if we move into an international federation approach.

Flagging for awareness.

AC:

• Set proper upper bound

Risk: We are not likely to reach this limit. It will become an issue if we decide to join an international federation.

[maxneuvians]

Set proper upper bound