chore: prepare to release 0.2.17 (#2392)

# 0.2.17 (April 9, 2020)

### Fixes
- rt: bug in work-stealing queue (#2387) 

### Changes 
- rt: threadpool uses logical CPU count instead of physical by default
  (#2391)


Signed-off-by: Eliza Weisman <eliza@buoyant.io>