Index bounds builder should generate exact bounds for timestamps

The index bounds builder can generate exact bounds for timestamps, but it does not currently, because timestamps are not considered a simple BSON type: https://github.com/mongodb/mongo/blob/r3.4.0-rc0/src/mongo/db/query/index_bounds_builder.cpp#L60. This can cause us to do an unnecessary FETCH stage.

As part of this work, we should consider removing isSimpleType() from BSONElement, since it is only used within the query system.