DAP-12 removed (specified) per-task HPKE key requests. We don't have a reason to want per-task HPKE keys, especially if they require custom functionality, so we remove per-task HPKE key support. With this change, all Janus deployments must use "global" HPKE keys.
Also, minorly refactor HpkeKeypairCache; mostly, to maintain all state under a single mutex and to remove an unnecessary "view" type.
DAP-12 removed (specified) per-task HPKE key requests. We don't have a reason to want per-task HPKE keys, especially if they require custom functionality, so we remove per-task HPKE key support. With this change, all Janus deployments must use "global" HPKE keys.
Also, minorly refactor HpkeKeypairCache; mostly, to maintain all state under a single mutex and to remove an unnecessary "view" type.