Fix incorrect size_type cast to unsigned int

[yichenluan]

What is the purpose of the change

Change find result type from unsigned int to string::size_type in NameSpaceUtil.

Brief changelog

Change find result type from unsigned int to string::size_type in NameSpaceUtil.

Verifying this change

build and test

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[ifplusor]

duplicated with #143

[yichenluan]

duplicated with #143

All right, maybe merge #143 ASAP.