format segment name, close #365

[vpecinka]

For Swift storage and large objects numbers has to be padded with zero for correct segment order.

[k0ka]

Thank you for the PR.

Your current implementation has not obvious limit of 10000 segments. I guess we should either assert it and throw an exception or add another parameter $data['segmentCountFormat'] to createLargeObject with default value %05d.

I also want to add an integration test for this case.

[vpecinka]

added the config option segmentIndexFormat with default value %05d and fixed the unit test.

[vpecinka]

also added isValidSegmentIndexFormat() to check proper format (has to produce different values for given format and different values, plus result has to have same string length)