No 64bit integer plugin APIs for HTTP headers

There are APIs for unsigned int, but stuff like content-length should be done as 64-bit values, so we should add TSMimeHdrFieldValueUint64Get(), TSMimeHdrFieldValueUint64Set(), TSMimeHdrFieldValueUint64Insert() or Int64 versions of those APIs (since the mime_* stuff seems to have unsigned int, int and int64, but no uint64).