poll.module should not query the node table directly

Followup to #1616962: Replace $node->title with $node->label() noted by Berdir: poll_page() directly queries the {node} table to built its list of polls, but should be using EFQ/entity_load instead, in order to obtain classed objects not that entities derive from Entity. When such nodes are passed to methods, they will be missing the expected EntityInterface methods.