Translate mongodb matchers into bson query to improve performance

[proxeus-dev]

Translate mongodb matchers into bson query to improve performance

The Select function of the mongodb implementation use matchers instead of bson queries.

Actual behaviour

The function select function in storage/dataase/db/mongo.go should use bson instead of matchers. Replace the matchers with bson. Current implementation:

func (s *MongoShim) Select(matchers ...q.Matcher) Query {
	// TODO: translate matchers into bson query to improve performance
	return &MongoQuery{matcher: q.And(matchers...), s: s}
}

See also #218