Fix circular reference between CBLUnsavedRevision and CBLAttachment

- As CBLUnsavedRevision keeps CBLAttachment objects in an array so setting the revision (self) object to the CBLAttachment objects will cause circular reference memory leak.

- Instead of setting the revision (self) object when adding a CBLAttachment to the CBLUnsavedRevision object, overiding the attachmentNamed: method and creating a new CBLAttachment object and adding a revision object there.

Source/API/CBLAttachment.m

@@ -72,7 +72,7 @@ - (instancetype) _initWithContentType: (NSString*)contentType
 }
 
 
-@synthesize revision=_rev, name=_name, metadata=_metadata;
+@synthesize revision=_rev, name=_name, body = _body, metadata=_metadata;
 
 
 - (NSString *)description

Source/API/CBLRevision.m

@@ -367,12 +367,28 @@ - (void) _addAttachment: (CBLAttachment*)attachment named: (NSString*)name {
     [atts setValue: attachment forKey: name];
     _properties[@"_attachments"] = atts;
     attachment.name = name;
-    attachment.revision = self;
+
+    // NOTE: Not setting the revision to the attachment object (attachment.revision = self) as
+    // [1] The UnsavedRevision object is not used during save operation
+    // [2] Setting the UnsavedRevision object here will cause the circular reference memory leak.
+    // The revision object will be set in the overridden attachmentName: method.
 }
 
 - (void) removeAttachmentNamed: (NSString*)name {
     [self _addAttachment: nil named: name];
 }
 
+- (CBLAttachment*) attachmentNamed: (NSString*)name {
+    CBLAttachment* base = self.attachmentMetadata[name];
+    if (base) {
+        CBLAttachment* attachment = [[CBLAttachment alloc] _initWithContentType: base.contentType
+                                                                           body: base.body];
+        attachment.name = name;
+        attachment.revision = self;
+        return attachment;
+    }
+    return nil;
+}
+
 
 @end

Source/API/CouchbaseLitePrivate.h

@@ -116,6 +116,7 @@
                              intoDatabase: (CBLDatabase*)database   __attribute__((nonnull(2)));
 @property (readwrite, copy) NSString* name;
 @property (readwrite, retain) CBLRevision* revision;
+@property (readonly) id body;
 @end