Moved read_t::use_snapshot to protocol.cc.
Made read_t::use_snapshot use a static_visitor for its implementation.
Made read_t::use_snapshot return true for intersecting_geo_read_t, nearest_geo_read_t, and distribution_read_t.
Adjusted read_t::use_snapshot comment.
Merge branch 'next' into sam_3238
Merge branch 'sam_3238' into next