R1:d450a088906e

Support HMAC+SHA256 with automatic key generation and management

Summary:
Ref T12509. This adds support for HMAC+SHA256 (instead of HMAC+SHA1). Although HMAC+SHA1 is not currently broken in any sense, SHA1 has a well-known collision and it's good to look at moving away from HMAC+SHA1.

The new mechanism also automatically generates and stores HMAC keys.

Currently, HMAC keys largely use a per-install constant defined in `security.hmac-key`. In theory this can be changed, but in practice essentially no…