feature: implements cursors in the EntityStreams API (resolved gh-242)

[bsbodden]

Implements the cursor API as supported by RediSearch using the Spring Framework Slice construct.
The test below creates a SearchStream and gets the initial "Slice" by usingloadAll() and toList. The overloaded toList takes a Spring PageRequest and the target class for the Slice.

The aggregation is lazily executed when the underlying slice is interrogated in any form that would require executing it. The case below will be in the while-loop hasContent() call. Subsequent slices can be obtained the "Spring Way" by passing the page's nextPageable:

@Test void testManagedCursorSession() {
    int pageSize = 45;
    SearchStream<Game> searchStream = entityStream.of(Game.class);

    Slice<Game> page = searchStream //
        .loadAll()
        .limit(300)
        .toList(PageRequest.ofSize(pageSize), Game.class);

    // loop through the slices using the SearchStream.getSlice method passing the next page request
    // obtained from the current page
    Map<Integer, Integer> pageCounts = new HashMap<>();
    while (page.hasContent()) {
      List<Game> contents = page.getContent();
      // collect the page counts
      pageCounts.put(page.getNumber(), contents.size());
      page = page.hasNext() ? searchStream.getSlice(page.nextPageable()) : Page.empty();
    }

    // assert the page counts
    assertAll("page counts",
        () -> assertEquals(pageSize, pageCounts.get(0)),
        () -> assertEquals(pageSize, pageCounts.get(1)),
        () -> assertEquals(pageSize, pageCounts.get(2)),
        () -> assertEquals(pageSize, pageCounts.get(3)),
        () -> assertEquals(pageSize, pageCounts.get(4)),
        () -> assertEquals(pageSize, pageCounts.get(5)),
        () -> assertEquals(30, pageCounts.get(6))
    );
  }