Made persistent_file_t use cache_conn_t.
Switched assertion order in txn_t::help_construct.
Made get_btree_superblock_and_txn not take a slice_t.
Made get_btree_superblock_and_txn_for_reading take a cache_t.
Made get_btree_superblock_and_txn_for_backfilling not take a btree_slice_t.
Made less use of indirect access through the btree_slice_t in btree_store.cc.
Made btree_store_t have a personal cache_conn_t.
Made all txn_t construction use a cache_conn_t.