Reuse the daemon RPC connection for the whole request

[jfpardy]

Since #644 the sync RPC helper closes the daemon
client after every call. So the second syncRpcCall() of a request, and every
one after it, dies with Cannot write to JSON-RPC socket.

Problem

close() nulls $connection but leaves $pendingConnection holding a promise
for it, because connect() repopulates it right after $connected clears it.
The next connection() hands back that promise, resolves a connection that's
already closed, and rejects at JsonRpcConnection::sendRequest()'s writability
guard.

The daemon status page shows this behaviour. VMware API Connections renders fine,
then Pending HTTP Requests stops working and log level control stops displaying(#647).

Clearing $pendingConnection fixes the rejection, but then every call still
opens its own connection. I tried that first, and on my test deployment the daemon
would only answer a single status page load per restart. Going back to one
connection per request skips the reconnect entirely. The stream watcher from
#644 only has to be gone before the loop could next run, and await() has
already stopped the loop by then, so React's shutdown handler leaves it alone.

Changes

• Share the RemoteClient across all RPC calls of a request and close it in a
  shutdown function, instead of a finally after every call.
• Reset $pendingConnection in RemoteClient::close() too, outside the if,
  since a failed connect leaves $connection null while the rejected promise
  sticks around.

closes #647

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