Support Market List Service (//blp/mktlist) requests

[jonfreedman]

From section 5.9 of the Core Developer Guide:

The Market List Service (//blp/mktlist) is used to perform two types of list data operations. The first is to subscribe to of instruments, known as “chains,” using the “chain” (i.e., //blp/mktlist/chain). The second is to a list of all the instruments that match a given topic key using the “secids” (i.e., /mktlist/secids). The //blp/mktlist service is available to both BPS (Bloomberg Professional service) and NONBPS .