Fix off-by-one error in continuation point calculation.

[alexander-travov]

I believe there is an off-by-one error in calculation of continuation point timestamp.

[alexander-travov]

For example I have a var on server with history of values [0, 1, 2, 3, 4, 5].
When I read history of that variable in portions of maxlen=3 with the use of continuation point, I get: [0, 1, 2], [4, 5]. The value 3 is never returned.

With the fix I get: [0, 1, 2], [3, 4, 5] as expected.

[alexander-travov]

Same error in FreeOpcUa/python-opcua#1494