Better logging if change tracker fails HTTP auth

If TDSocketChangeTracker gets a 401 or 407 and can't find a credential, or the credential is rejected,
it now logs a message of the form
    TDSocketChangeTracker[...]: HTTP auth failed; sent Authorization: %@  ;  got WWW-Authenticate: %@
showing the Authorization header it sent (if any, else null) and the WWW-Authenticate response header.
These headers are also returned in the NSError as keys "HTTPAuthorization" and "HTTPAuthenticateHeader"
(although this is of limited use since this info won't make it all the way up to the public replication API, yet.)
This should help in diagnosing issue #242.