Move caching logic into oauthdb.getClientInfo

[data-sync-user]

In lib/senders/oauth_client_info.js we currently have some caching logic wrapped around a call to oauthdb.getClientInfo, to avoid hitting the oauth-server db every time we want to look up basic oauth client metadata.

As seen in mozilla/fxa-auth-server#2985, we're growing additional calls to oauthdb.getClientInfo that would also benefit from this caching, but don't get it.

Let's move the caching logic down into oauthdb.getClientInfo or some low-level wrapper around it, so it can be shared by more components.

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[cknowles-admin]

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