Don't modify keyPath until right before recursive renderNode call

Currently, if a component suspends, the keyPath has already been modified to
include the identity of the component itself; the path is set before the
component body is called (akin to the begin phase in Fiber). An accidental
consequence is that when the promise resolves and component is retried, the
identity gets appended to the keyPath again, leading to a duplicate node in
the path.

To address this, we should only modify contexts after any code that may suspend.
For maximum safety, this should occur as late as possible: right before the
recursive renderNode call, before the children are rendered.