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/namespaced.go

@@ -17,13 +17,13 @@ import (
 // NamespacedKV is a simple key-value store using a specific namespace within
 // a leveldb.
 type NamespacedKV struct {
-	db     *leveldb.DB
+	db     *Instance
 	prefix []byte
 }
 
 // NewNamespacedKV returns a new NamespacedKV that lives in the namespace
 // specified by the prefix.
-func NewNamespacedKV(db *leveldb.DB, prefix string) *NamespacedKV {
+func NewNamespacedKV(db *Instance, prefix string) *NamespacedKV {
 	return &NamespacedKV{
 		db:     db,
 		prefix: []byte(prefix),