Make authentication drivers Thread-safe

Setting and getting authentication state was Thread-unsafe so that
caused 'can't add a new key into hash during iteration' in race
condition.  Introduce Mutexes.  Closes #200.
Remove debugging stuff.

Additional fix for #200
Avoid unnecessary conn retries for OAuth error

Make challenge() in OAuth handler always returns false to avoid connection
retry which should not work in OAuth authentication context.  This method just
remember URL (nil means 'any') for the next connection.  Normally OAuthClient
handles this correctly but see how it uses when you need to use this class
directly.  Closes #203.
Suppress return value of SSLConfig#change_notify

Some instance_methods of SSLConfig returns the returning value of
change_notify unintentionally.  No one should depend on this undefined
value. (ex. SSLConfig#add_trust_file)  It was a Hash (almost empty) and
now it's nil.
bump version