Change locking for Scorecard interface

[raghavkaul]

This PR makes the following changes:

1. Initializes scClients once globally
2. In Scorecard.Get, use Lock()/Unlock() for reading from the scClients map. This prevents two threads from independently reading the value, both deciding that there's no scClient for their repo, and independently (but not concurrently) creating clients. Although they don't concurrently modify the map, thread 2 will overwrite the scClient created by thread 1.
3. In Scorecard.Close, use Lock()/Unlock() instead of the R-variants. This prevents the case where two threads Close() both reach the if !ok line with ok == true, and then independently (but not concurrently) attempting ScRepoClient.Close and delete.

This should reduce overall repeated work by making the critical sections larger.