CXX-206 Backport server 2.6.0..2.7.0 changes

Files merged manually or fixed up:
 SConstruct
 src/mongo/bson/bsonobj.h
 src/mongo/bson/optime.cpp
 src/mongo/bson/optime.h
 src/mongo/client/connpool.h
 src/mongo/client/dbclientinterface.h
 src/mongo/db/repl/member_state.h
 src/mongo/dbtests/mock/mock_replica_set.cpp
 src/mongo/util/assert_util.h
 src/mongo/util/log.h
 src/mongo/util/time_support.h
CXX-185 - aggregation helper

Added in an aggregation helper and shims for the DBClientCursor to allow
for the special semantics used for return values.