Having large numbers of networks can cause node allocation to fail

Since 9fa9ce1b5280f3f5d3ab659e3bd88cda0fb305a3, nodes are now allocated with a network attachment for every network.

Unfortunately, this can cause the size of a Node object to exceed the maximum raft message size, which prevents it from being committed to the object store.

From moby/moby#36792

Forgot to mention the same error was triggered by a different action than the one initially reported on this issue, in my case it appeared when creating ~1700 networks and deploying one service on each network.

Jun 01 00:39:52 ip-172-16-0-128 dockerd[1284]: time="2018-06-01T00:39:52.220386603Z" level=error msg="Failed to commit allocation of network resources for node rul9pnxcc2hpj3o7eya1redpk" error="raft: raft message is too large and can't be sent" module=node node.id=281crvu

There are many possible fixes, including disallowing too many attachments, but one way or another we can't wind up with raft messages too large like this.

/cc @ctelfer

Start with commit 9fa9ce1b5280f3f5d3ab659e3bd88cda0fb305a3 and trace node allocation through the object store and Raft message handling. Reproduce the failure with many network attachments and inspect the reported maximum-message error. Done means node allocation cannot produce Raft messages that exceed the allowed size.