Bug 6158: Optimization of if-feaures resolution during SchemaContext assembly

Bug 6158: Optimization of if-feaures resolution during SchemaContext assembly

It is unnecessary to check both declared and effective substatements everytime,
because effective substatements are empty at the beginning and they are filled
subsequently by copying of declared. Therefore this patch check whether
a feature is supported only for declarted statements during their build and
also creation of their copy.
In addition this patch introduces ALL_FEATURES predicate constant. The constant
is passed as argument into default SchemaContext build, and so we can simply
return true and skip rest of areFeaturesSupported method.
Moreover the result of areFeaturesSupported method is cached and so it is
not computed more times for any statement.

Change-Id: If8c87975712b65f9f28b640ac0b634576e08c191
Signed-off-by: Peter Kajsa <pkajsa@cisco.com>