Bug 1056523 - Ensure sync credentials are reset during reauth flow. r=markh

This patch addresses a bug in the following scenario:

User has browser 1 connected to sync and open, and resets her password
on browser 2. Eventually the browser detects the need to reauthenticate
the user, and prompts the user. When the user entered her new password,
the browserid_identity module failed to re-derive a new syncKeyBundle
from the new password and happily used the old one. Chaos ensued.

This patch mitigate the problem by calling resetCredentials at the
start of initializeWithCurrentIdentity(), which will clear the
syncKeyBundle, along with other credentials. Previously this
function was only resetting this._shouldHaveSyncKeyBundle.

I also removed a misleading comment about the syncKeyBundle being
cleared when it wasn't.