SPY-178: KetamaNodeLocator#getReadonlyCopy uses unsafe operations.

Motivation
----------
Users reported java.lang.UnsupportedOperationException when using the
KetamaNodeLocator#getReadonlyCopy() method. It turns out the error
only shows up when the JVM is run with the -XX:+AggressiveOpts flag.

Modifications
-------------
Changing the iterator to properly update the TreeMap while iterating
fixes the problem.

Result
------
Correct behavior for KetamaNodeLocator#getReadonlyCopy(), even when
the -XX:+AggressiveOpts flag is enabled.

Change-Id: I6e7f09892687b8896c9b295a94fb473857ec212f
Reviewed-on: http://review.couchbase.org/43040
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>