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/virtualmtime.go

@@ -9,8 +9,6 @@ package db
 import (
 	"fmt"
 	"time"
-
-	"github.com/syndtr/goleveldb/leveldb"
 )
 
 // This type encapsulates a repository of mtimes for platforms where file mtimes
@@ -25,7 +23,7 @@ type VirtualMtimeRepo struct {
 	ns *NamespacedKV
 }
 
-func NewVirtualMtimeRepo(ldb *leveldb.DB, folder string) *VirtualMtimeRepo {
+func NewVirtualMtimeRepo(ldb *Instance, folder string) *VirtualMtimeRepo {
 	prefix := string(KeyTypeVirtualMtime) + folder
 
 	return &VirtualMtimeRepo{