call .read() only once when dataset do not have nodata

[vincentsarago]

For a dataset that has a mask or and alpha band (by default every data that don't have Nodata set will have an alpha/mask band within the WarpedVRT instance) we were doing two read(), one for the data, another one for the mask. This PR changes the behavior and performs only one read() and then split the data and the mask to two arrays.

### Before 
$ python tiprofile.py https://rio-tiler-dev.s3.amazonaws.com/data/m_3209542_se_15_060_20181115_20190222.tif --tile 13-1916-3319 --config GDAL_DISABLE_READDIR_ON_OPEN=EMPTY_DIR
         193901 function calls (193178 primitive calls) in 0.978 seconds

   Ordered by: internal time, cumulative time
   List reduced from 635 to 10 due to restriction <10>

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.571    0.571    0.571    0.571 __init__.py:56(open)
        2    0.118    0.059    0.118    0.059 {method 'read' of 'rasterio._warp.WarpedVRTReaderBase' objects}
 
### Now
$ python tiprofile.py https://rio-tiler-dev.s3.amazonaws.com/data/m_3209542_se_15_060_20181115_20190222.tif --tile 13-1916-3319 --config GDAL_DISABLE_READDIR_ON_OPEN=EMPTY_DIR       
         193801 function calls (193078 primitive calls) in 0.885 seconds

   Ordered by: internal time, cumulative time
   List reduced from 636 to 10 due to restriction <10>

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.382    0.382    0.383    0.383 __init__.py:56(open)
        1    0.048    0.048    0.048    0.048 {method 'read' of 'rasterio._warp.WarpedVRTReaderBase' objects}

☝️ this is a profiling results that show that the method 'read' of 'rasterio._warp.WarpedVRTReaderBase' objects was called twice and only once in the new version.