CID encryption and rotation

[kazuho]

Using a structure like below:

struct PerServerCID {
    uint8_t version;           // hard-coded to zero
    uint16_t process_id : 4;   // for forwarding packets to the correct process
    uint16_t thread_id : 12;   // for forwarding packet to the correct thread
    uint32_t conn_id;          // unique ID per connection
    uint8_t  path_id;          // path id
};

struct CIDPlaintext {
    uint32_t     server_id[4];  // for forwarding packets to the correct server
    uint32_t     zero;
    PerServerCID per_server;
};

CID = AES_ECB(key_id || AES_ECB(key, CIDPlaintext))

Design requirements:

• use 4-byte server ID (to store internal IP address of each server)
• 4-byte zero to detect bogus CIDs
• 8-byte per-server CID to support 9-byte CID when running on a single server (using 64-bit cipher)
• encode path_id directly, to avoid adding multiple entries to the CID -> connection hashmap

[kazuho]

partially closed by #85, leftovers to be handled in #92.