Consistent handling of link count version and timestamp

Summary:
We don't handle version and timestamp for link count table
consistently between deleteLink and addLink.  This change
standardises so that time is timestamp of last update,
and version is updated on every change.

Test Plan: ran MySqlLinkTest

Reviewers: dhruba, emayanke

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D10611