Pre-cache child node set

Tracing has shown this method gets called ~5.7M times, contributing to
some 10% bootup time. Pre-cache the set of nodes to speed things up.

Change-Id: I20bf25b55a4c52a725c160d7d52c6cdd8ce4dfa0
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Pre-cache child node set"