Marker pcaps are written by uBridge on the compute that hosts the capture
node. The tag replay only looked in the project folder on the controller.
So a source from a remote compute showed count 0 and no frames, with no
error. Its frame detail returned 404 and said nothing ever matched (#3010).
Each source now gets its pcap through the compute of its capture node.
Local files are read as before. Remote files are fetched over HTTP on
every acquire into a small cache (32 idle files, cleared at shutdown).
A file is replaced only when its bytes changed. This keeps the old mtime,
so the sharkd session for that file stays alive.
When a compute cannot be reached, its source now shows an error message
in the timeline instead of just showing nothing. The detail endpoint now
returns an error instead of 404.
