This should protect from possible data loss when `vmstorage` is stopped while the packet is sent from `vminsert`.
This commit switches to new protocol between vminsert and vmstorage, which is incompatible
with the previous protocol. So it is required that both vminsert and vmstorage nodes are updated.
The conn can be missing or already closed during the call to storageNode.closeBrokenConn.
Prevent `nil pointer dereference` panic by verifying whether the conn is already closed.
Thanks to @CH-anhngo for reporting the issue.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/189
* Spread load evenly among remaining healthy nodes instead of hammering
the next node after the unhealthy node.
* Make sure that the packet is flushed to storage node before returning success.
Previously packets could stay in local buffers and thus lost on connection errors.
* Keep rows in the limited memory when all the storage nodes are unhealthy.
