Locking doesn't take into account availability of children

[tomgreenfield]

In adaptModel.js, the locking functionality doen't check for "_isAvailable": true when setting _isLocked.

Would be useful to include a new function getAvailableChildren() since a collection is already requested in three other places in the file.

[oliverfoster]

+1

[moloko]

done in 2f43c2c