fix: cross-pod state for hex tile builds (#146)

* docs: spec for cross-pod hex tile build state

* docs: spec switch to no-delete marker scheme (no boto3 dep)

* docs: implementation plan for hex tile cross-pod state

* tiles: add lock.json marker helpers for cross-pod build state

* tiles: add failed.json marker helpers

* server: get_hex_tile_status checks failed.json before _jobs

* server: register_hex_tiles short-circuits when failed.json exists

* server: _do_build writes failed.json on build exception

* server: register_hex_tiles writes lock.json + dedups across pods

* server: get_hex_tile_status long-polls S3 when build is on another pod

Author: cboettig