Simplify the server side of cold standby

With the changes introduced in OAK-4803, it would be nice to keep the previous symmetry between the client and server and remove thus the FileStore reference from the latter.

Per frm's suggestion from one of the comments in OAK-4803:

In the end, these are the only three lines where the FileStore is used in the server, which already suggests that this separation of concerns exists - at least at the level of the handlers.

p.addLast(new GetHeadRequestHandler(new DefaultStandbyHeadReader(store)));
p.addLast(new GetSegmentRequestHandler(new DefaultStandbySegmentReader(store)));
p.addLast(new GetBlobRequestHandler(new DefaultStandbyBlobReader(store)));