rpc: rework how responses are written back via HTTP (#7575)

Add writeRPCResponse and writeHTTPResponse helpers, that handle the way RPC
responses are written to HTTP replies. These replace the exported helpers.

Visible effects:

- JSON results are now marshaled without indentation.
- HTTP status codes are now normalized.
- Cache control headers are no longer set.

Details:

- When writing a response to a URL (GET) request, do not marshal the whole
  JSON-RPC object into the body, only encode the result or the error object.
  This is a user-visible change.

- Do not change the HTTP status code for RPC errors. The RPC error already
  reports what went wrong, the HTTP status should only report problems with the
  HTTP transaction itself. This is a user-visible change.

- Encode JSON without indentation in POST response bodies. This is mainly cosmetic
  but saves quite a bit of response data. Indent is still applied to GET responses to make
  life easier for code examples.

- Remove an obsolete TODO about reporting an HTTP error on websocket upgrade.
  Nothing needed to change; the upgrader already reports an error.

- Report an HTTP error when starting the server loop fails.

- Improve logging for encoding errors.

- Log less aggressively.

test/app/kvstore_test.sh

@@ -57,7 +57,7 @@ echo "... testing query with /abci_query 2"
 
 # we should be able to look up the key
 RESPONSE=`curl -s "127.0.0.1:26657/abci_query?path=\"\"&data=$(toHex $KEY)&prove=false"`
-RESPONSE=`echo $RESPONSE | jq .result.response.log`
+RESPONSE=`echo $RESPONSE | jq .response.log`
 
 set +e
 A=`echo $RESPONSE | grep 'exists'`
@@ -70,7 +70,7 @@ set -e
 
 # we should not be able to look up the value
 RESPONSE=`curl -s "127.0.0.1:26657/abci_query?path=\"\"&data=$(toHex $VALUE)&prove=false"`
-RESPONSE=`echo $RESPONSE | jq .result.response.log`
+RESPONSE=`echo $RESPONSE | jq .response.log`
 set +e
 A=`echo $RESPONSE | grep 'exists'`
 if [[ $? == 0 ]]; then