Add build timestamps and add section timings to slack messages, and improve error handling

[VillePihlava]

Bot-posted Slack messages don't show a timestamp other than the normal slack message timestamp.
Timestamps and durations are now explicit in the message text.

What changed

• Slack messages now include explicit clock-time/duration info.
• Added a section-timing summary (main gulp tasks: cleanup, seeding,
  osm/dem, transit data, build graph, tests, GTFS fallback + rebuild,
  upload, deploy), posted as a thread reply when the build finishes.
• Centralized all build-termination logic (status message, summary post,
  exit code) into a single finalizeBuild() in builderUtils.js, used by
  both normal completion/failure paths in Update.js and the crash
  safety-net in index.js.
• Simplified update() to a single try/catch, each branch ending in one
  finalizeBuild() call.
• Extracted formatUtils.js (formatClockTime, formatDuration,
  getDateStringForDockerTag) and renamed buildTimer.js →
  timerUtils.js for clearer module boundaries.
• Renamed buildStreetOnlyGraph/buildGraph/buildWithPrebuiltStreetGraph
  parameter name → routerId for clarity.

Verification

yarn test (prettier/eslint/config validation) and manual mocked-axios
dry runs of success, partial-fallback, abort, generic-error, and
crash-handler paths.