We should pass around db.Instance instead of leveldb.DB

We're going to need the db.Instance to keep some state, and for that to
work we need the same one passed around everywhere. Hence this moves the
leveldb-specific file opening stuff into the db package and exports the
dbInstance type.

lib/db/blockmap.go

@@ -29,11 +29,11 @@ var blockFinder *BlockFinder
 const maxBatchSize = 256 << 10
 
 type BlockMap struct {
-	db     *leveldb.DB
+	db     *Instance
 	folder string
 }
 
-func NewBlockMap(db *leveldb.DB, folder string) *BlockMap {
+func NewBlockMap(db *Instance, folder string) *BlockMap {
 	return &BlockMap{
 		db:     db,
 		folder: folder,
@@ -146,10 +146,10 @@ func (m *BlockMap) blockKeyInto(o, hash []byte, file string) []byte {
 }
 
 type BlockFinder struct {
-	db *leveldb.DB
+	db *Instance
 }
 
-func NewBlockFinder(db *leveldb.DB) *BlockFinder {
+func NewBlockFinder(db *Instance) *BlockFinder {
 	if blockFinder != nil {
 		return blockFinder
 	}