SERVER-2193 Support for multi field spares index

Multi field sparse index can work in two modes controlled by the new
sparsePolicy option. When sparsePolicy is not set at all or set to the
string "exclude" the rule for the sparse index is: "exclude a document
from the index if all index keys are missing from the document". This
is the default.

If, on the other hand, sparsePolicy is set to "include" the rule for the
index is: "only include the document in the index if all index keys are
present in the document".

The file jstests/index_sparse3.js is a copy of the existing
jstests/index_sparse2.js and tests the "include" policy in the same way
as index_sparse2.js test the "exclude" policy.