[MD] Cache reader in image mosaic walk and granule initialization

[aaime]

When playing with mosaics made of NetCDF harvesting and getting all granules is extremely slow because the reader is created again each time.
Cache the last reader and check if it can be reused.

[simboss]

I have taken a stab at this. It does not seem to be extremely easy as the underlying reader should be probably share with the parent ImageMosaic reader (which would probably speed up the reading for it as well).