Change strand executor segment size to 64
Change strand executor segment size to 64

Summary: This is an analogous change to D6665918 (https://github.com/facebook/folly/commit/4591832b4885f166236acee689dd2b11e3b3135b), which should reduce RSS variance due to extra allocated queue segments.

Reviewed By: yfeldblum

Differential Revision: D26770721

fbshipit-source-id: 7e83aaae1d5030c5ed50b34fae06236f0ead8ac1

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly