fixes #613 SERVER-8173: add a new IndexPathSet class to make checking if a mod will udpate index more reliable

Conflicts:
	src/mongo/SConscript
	src/mongo/db/namespace_details-inl.h
	src/mongo/db/namespace_details.cpp
	src/mongo/db/namespace_details.h
	src/mongo/db/ops/update.cpp
	src/mongo/db/ops/update_internal.cpp
	src/mongo/db/ops/update_internal.h