R1:1d34238dc945

Upgrade sessions digests to HMAC256, retaining compatibility with old digests

Summary:
Ref T13222. Ref T13225. We store a digest of the session key in the session table (not the session key itself) so that users with access to this table can't easily steal sessions by just setting their cookies to values from the table.

Users with access to the database can //probably// do plenty of other bad stuff (e.g., T13134 mentions digesting Conduit tokens) but there's very little cost to storing digests instead of…