Log on reconfiguration start
Log on reconfiguration start

Summary: useful to measure reconfiguration time and track bugs related to reconfiguration.

Test Plan: unit tests

Reviewed By: andreazevedo

Differential Revision: D2659678

fb-gh-sync-id: 64a54fd3a281bc03db940f6b3a1f06711657954e

facebook/mcrouter,nvaller/mcrouter,facebook/mcrouter,facebook/mcrouter,synecdoche/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,facebook/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,nvaller/mcrouter