Improved child id caching

Fix a bug triggered by calling setPlaceholders multiple times for
children of the same resource but with different sort orders.
Tweak caching to hopefully prevent PHP warnings

Hopefully prevents "Argument 1 passed to xPDOObject::load() must be an
instance of xPDO..." warnings.
Much faster child IDs lookup
One more  caching tweak
Revert "Tweak caching to hopefully prevent PHP warnings"

This reverts commit 43a7582b92d413b29737fe8cdcb2f1457f40448b.
Minor performance enhancements
Bump version to 1.2.1
Set any empty placeholders to ''

This allows them to be fully parsed and cached.
1.2.1 Changelog
Merge branch 'dev'