Deprecate MODIFY_EXCLUSIVE #865

The dev version of libtiledb has deprecated the flag MODIFY_EXCLUSIVE (TileDB-Inc/TileDB#5721). For the short-term I have fixed the nightly TileDB-R builds (#863) by modifying a single test (#864). Longer-term we will need to deprecate the use of MODIFY_EXCLUSIVE throughout the package.

Is it sufficient to send a warning if type == "MODIFY_EXCLUSIVE" and then change type to WRITE?

cc: @teo-tsirpanis, @ihnorton