sln-pipe doesn't detect disconnections

[btrask]

sln-pipe is not particularly robust currently. It detects disconnections on the upstream (sending) side, but not on the downstream (receiving) side.

I'm thinking we should add another API endpoint for a simple heartbeat over HTTP. This would allow HTTP clients to detect disconnections more easily.

For now, anyone wanting to do a "Call Me Maybe" style test of StrongLink syncing in the face of network partitions should use SLNPull, the internal sync system. Unfortunately it can't do partial syncs currently.