fix(template-outlet): memory leak for cached templates

[pmoleri]

When a TemplateOutlet instance was being destroyed all its locally cached views were not being destroyed.

The parent component (grid) was attempting to clean up all the template outlets on the its destroy. But, this approach is weak because TemplateOutlets could have been destroyed along the way (resize, groupby, etc), so all those caches would have never been destroyed.

The new strategy:

• uniquely identified views (templateID.id) are meant to be managed by the parent component, so now the parent component destroys them directly and the template outlet never caches them locally.
• other templates (local) are destroyed by the template outlet destroy.

Example bug scenario in the old code:

• Grid renders 20 rows
• Group by is run, some of the template outlets change to summary template, detaching their old row viewref which is kept in the cache.
• Then, some operation removes templates outlets, e.g. grid resize, page size change, filter data.
• Those template-outlet instances get destroyed but .destroy() is not called on their cached/detached views. Angular still holds a reference to them permanently.
• Grid gets destroyed. The remaining template-outlet instances are cleaned up correctly. But previous undestroyed ones are still lingering.

Repro:

• Go to: https://pmoleri.github.io/grid-leak/grid
• Don't open the devtools yet
• Drag "Title" column to group by it
• Change page size to 5
• Click on "Show" chekbox to destroy the grid
• Open Devtools
• In Memory tab Take a snapshot
• Select "Object retained by detached DOM nodes"
• See the leaked <igx-grid-row> elements

Note that this repro is based on 19.1.5 which doesn't contain other fixes. So, you'll find also other leaks.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them