Implement lazy iteration (ForEach) over collections

See #761

aptly had a concept of loading small amount of info per each object
into memory once collection is accessed for the first time.

This might have simplified some operations, but it doesn't scale well
with huge aptly databases.

This is just intermediate step towards better memory management -
list of objects is not loaded unless some method is called.
`ForEach` method (mainly used in cleanup) is reimplemented to
iterate over database without ever loading all the objects into memory.

Memory was even worse with previous approach, as for each item usually
`LoadComplete()` is called, which pulls even more data into memory
and item stays in memory till the end of the iteration as it is referenced
from `collection.list`.

For the subsequent PR: reimplement `ByUUID()` and probably other methods
to avoid loading all the items into memory, at least for all the collecitons
except for published repos. When published repository is being loaded, it
might pull source local repo which in turn would trigger loading for all the
local repos which is not acceptable.

deb/remote.go

@@ -660,28 +660,34 @@ type RemoteRepoCollection struct {
 
 // NewRemoteRepoCollection loads RemoteRepos from DB and makes up collection
 func NewRemoteRepoCollection(db database.Storage) *RemoteRepoCollection {
-	result := &RemoteRepoCollection{
+	return &RemoteRepoCollection{
 		RWMutex: &sync.RWMutex{},
 		db:      db,
 	}
+}
 
-	blobs := db.FetchByPrefix([]byte("R"))
-	result.list = make([]*RemoteRepo, 0, len(blobs))
+func (collection *RemoteRepoCollection) loadList() {
+	if collection.list != nil {
+		return
+	}
+
+	blobs := collection.db.FetchByPrefix([]byte("R"))
+	collection.list = make([]*RemoteRepo, 0, len(blobs))
 
 	for _, blob := range blobs {
 		r := &RemoteRepo{}
 		if err := r.Decode(blob); err != nil {
 			log.Printf("Error decoding mirror: %s\n", err)
 		} else {
-			result.list = append(result.list, r)
+			collection.list = append(collection.list, r)
 		}
 	}
-
-	return result
 }
 
 // Add appends new repo to collection and saves it
 func (collection *RemoteRepoCollection) Add(repo *RemoteRepo) error {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.Name == repo.Name {
 			return fmt.Errorf("mirror with name %s already exists", repo.Name)
@@ -728,6 +734,8 @@ func (collection *RemoteRepoCollection) LoadComplete(repo *RemoteRepo) error {
 
 // ByName looks up repository by name
 func (collection *RemoteRepoCollection) ByName(name string) (*RemoteRepo, error) {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.Name == name {
 			return r, nil
@@ -738,6 +746,8 @@ func (collection *RemoteRepoCollection) ByName(name string) (*RemoteRepo, error)
 
 // ByUUID looks up repository by uuid
 func (collection *RemoteRepoCollection) ByUUID(uuid string) (*RemoteRepo, error) {
+	collection.loadList()
+
 	for _, r := range collection.list {
 		if r.UUID == uuid {
 			return r, nil
@@ -748,23 +758,28 @@ func (collection *RemoteRepoCollection) ByUUID(uuid string) (*RemoteRepo, error)
 
 // ForEach runs method for each repository
 func (collection *RemoteRepoCollection) ForEach(handler func(*RemoteRepo) error) error {
-	var err error
-	for _, r := range collection.list {
-		err = handler(r)
-		if err != nil {
-			return err
+	return collection.db.ProcessByPrefix([]byte("R"), func(key, blob []byte) error {
+		r := &RemoteRepo{}
+		if err := r.Decode(blob); err != nil {
+			log.Printf("Error decoding mirror: %s\n", err)
+			return nil
 		}
-	}
-	return err
+
+		return handler(r)
+	})
 }
 
 // Len returns number of remote repos
 func (collection *RemoteRepoCollection) Len() int {
+	collection.loadList()
+
 	return len(collection.list)
 }
 
 // Drop removes remote repo from collection
 func (collection *RemoteRepoCollection) Drop(repo *RemoteRepo) error {
+	collection.loadList()
+
 	repoPosition := -1
 
 	for i, r := range collection.list {