subfolders missing from the folder tree

Problem/Motivation

subfolders missing from the folder tree when the pager patch from #3134212 is applied

Steps to reproduce

Install IMCE 3.1.5 and apply the patch from #3134212.
Use a stream wrapper such as public:// or s3:// with a folder holding more than 50 files and several subfolders.
Open IMCE, for example from CKEditor.
The tree shows the folder as loading, or lists images without the subfolders. Pager navigation can also show stale or missing items

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes