remove from DNN module headers

* core(test): merge hang test

* core(merge/split): fix intrin optimization

fixed call open(int id) failed

All vx_call() must be wrapped into own simd128/simd256/simd512 namespace

```
namespace CV__SIMD_NAMESPACE {
... vx_call declaration is here ...
}
```

- use '3.4.x' cache name for current maintenance series (there are no serious changes between releases)
- message is shown only once during creation of new cache directory
- use OPENCV_CACHE_SHOW_CLEANUP_MESSAGE=0 to hide this warning

imgproc: update cornerSubPix documentation

Fix for SSE2 intrinsics problem in the part of saturation arithmetic processing during 32s->16u packed conversion -
for some big negative values less than -INT_MAX+32767 the sign of the numbers is lost due to overflow that leads to
incorrect saturation to MAX value, instead of zero.
The issue is not reproduced with CV_ENABLED_INTRINSICS=OFF

- tests for both SIMD128 / SIMD256
- different dispatched + baseline(SIMD128) intrinsics