[hf] healthchecks timeout after 15 minutes (but still run every 5 min)

motivation:

• healthchecks are acting up since moving them to 5 min intervals w/ 5 min timeout
• this makes sense, b/c on busy channels, messages can often take over 5 minutes to transmit, making it impossible to transmit healthchecks during that window if they are busy
• so: let's try running healthchecks every 5 min, but giving them 15 min to time out. this would create a max window of 20 min downtime in the event of true failure (b/c we restart after 2 failed healthchecks)