tempfile.NamedTemporaryFile behaving as /vsimem and eating all the machine memory


While working on https://github.com/cogeotiff/rio-cogeo/pull/75 we noticed strange behaviors with `vsimem` driver (this could be a GDAL but TBH). 

1. When using `
tempfile.NamedTemporaryFile()` Rasterio uses `vsimem` driver

with tempfile.NamedTemporaryFile() as tmpfile:


   with rasterio.open(tmpfile, "w", **meta) as tmp_dst:


<tempfile._TemporaryFileWrapper object at 0x1151bb2e8>

<open DatasetWriter name='/vsimem/d26d4650-3010-4d39-8b0c-3d947b94f1d5.' mode='w+'>

Here I was expecting Rasterio/GDAL to behave as `tempfile` was a regular file.

2. When closing
 a `vsimem`  (`MemoryFile` or `tempfile`) we observe a huge memory surge when working with big images.


Tested on Mac OS and linux with python 3.7 (gdal 2.4 and 2.3) 


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