GTiff: improve performance of internal overview creation (fixes #1442)

[rouault]

Or multiple level external overview.

This was a long standing problem. The issue was that each overview level (this
also applies to mask) within a same TIFF file shared the same TIFF* handle.
When switching between overviews, the handle had to be reset to point to a
new TIFF IFD, and reloading of the StripOffset/StripByteCount/TileOffset/TileByteCount
was done. For really big files, those can be several ten to hundred of MB large.

Now, each overview (mask) owns its own TIFF handle, sharing the underlying file
pointer though. Some care is needed in the tifvsi layer regarding some write
optimizations, as well when creating overviews, we must take care of forcing
a reload of the main TIFF handle so that the in-memory TIFF structure points
correctly to the next IFD.

Test case:

from osgeo import gdal
gdal.GetDriverByName('GTiff').Create('large.tif', 100000, 100000, 1, options = ['TILED=YES', 'SPARSE_OK=YES'])

With this improvement:
$ cp large.tif input.tif; time gdaladdo input.tif 2 4 8 --debug on
real 0m54.913s

Without:
real 3m0.175s