Stopped storing slice in traversal_state_t.
Made traversal_state_t constructor not take a btree_slice_t.
Removed a bunch of btree_slice_t parameters.
Removed sindex_access_t parameter from sindex_erase_range.
Removed btree_slice_t parameter from rdb_erase_range.
Removed btree_slice_t parameter from rdb_distribution_get.
Removed some unused btree_slice_t params in memcache code.
Removed unused slice parameter from clear_sindex.
Replaced expose_buf().cache() calls with cache().