Fix blocking send in online Histo sender
- Fixes the sometime 40s-1min long waits when sending histos for CA or event builder (probable cause for jumps in TS count in Grafana)
- Should also fix the blocking wait at startup if no histogram server available if I believe https://libzmq.readthedocs.io/en/latest/zmq_msg_send.html
For info: ping @fweig @j.decuveland