v2.4.0

Externally visible changes since v2.3.9:

• Faster startup by scanning cache dir in parallel, and optimising the LRU index sorting.
• Make idle_timeout setting also apply when using https.
• Shutdown gracefully on sigint (ctrl-c) and sigterm.
• Exit immediately if either of the HTTP or gRPC servers fail.
• Fix AC key mangling for gRPC.
• Always allow gRPC health check requests, even without authentication. #590
• Fix cache file permissions, they shouldn't be world-writable.
• Avoid a bunch of corner case crashes.
• Use a docker base container image without openssl. #605
• Advertise support up to REAPI version 2.3.
• Allow setting local timezone for logs.
• Allow logging without timestamps.