Optimize out degree count query

[adsharma]

If your node table is sorted by primary key, we can avoid joins and CSR indptr to answer.
Note that you have to ensure the following invariant for correctness:

a.primary_key == a.rowid

So it's more than just sorting in ascending order.

Before:

--- 9. Out-degree for 100 nodes (id 1..100) ---
    run 1: 54.9 ms
    run 2: 54.7 ms
    run 3: 54.5 ms
    => best:  54.5 ms
    => worst: 54.9 ms
    => avg (warm): 54.8 ms
  Sum of out-degrees for 100 nodes: 500

after:

--- 9. Out-degree for 100 nodes (id 1..100) ---
  run 1: 28.1 ms
  run 2: 25.9 ms
  run 3: 26.2 ms
  => best:  25.9 ms
  => worst: 28.1 ms
  => avg (warm): 27.1 ms
Sum of out-degrees for 100 nodes: 500