Identify and fix "broken pipe" issue in RPC server

[str4d]

A follow-on from #2263:

HTTP connection to node 0 may fall into a state, during the few minutes it takes to process
loop above to create new addresses, that when z_sendmany is called with a large amount of
rpc data in recipients, the connection fails with a 'broken pipe' error. Making a RPC call
to node 0 before calling z_sendmany appears to fix this issue, perhaps putting the HTTP
connection into a good state to handle a large amount of data in recipients.

We need to figure out the root cause of this issue, given it appears to be with the RPC server rather than the RPC tests. Perhaps upstream has a PR already.

[str4d]

Also seen in #2759.