Bug 1059391 - Add pref to disable caching of Sync authentication credentials. r=markh

This introduces a debugging pref, "services.sync.debug.ignoreCachedAuthCredentials".
This flag allows testers to disable caching of authentication credentials
to make debugging of expired and revoked credentials easier. This will
help expedite any visble auth errors resulting from a expired or revoked
FxA session token, e.g., from resetting or changing the FxA password.
This pref is not set by default.