Update default cache lookup logic in ConfidentialClientApplication

[sangonzal]

• Client credential flow should by default look in the cache and try to use the cached access token. Internal design document
• On-behalf-of flow should by default look in the cache and try to use the cached access token. Internal design document

[chlowell]

@rayluo is this still relevant? I'm implementing OBO and wonder whether there are any gotchas or limitations in the current MSAL I should consider.

[rayluo]

For a different reason, and as a byproduct of #577, the first topic "client credential flow" is now changed to automatically look up tokens from cache.

The OBO flow remains unchanged, for now. The current calling pattern still utilize acquire_token_silent(). Such a calling pattern is useful in long-lived OBO apps.