lib/model: Remove bogus fields from connections API endpoint (fixes #8103) (#8104)

* lib/model: Remove bogus fields from connections API endpoint.

Switch the returned data type for the /rest/system/connections element
"total" to use only the Statistics struct.  The other fields of the
ConnectionInfo struct are not populated and misleading.

* Lowercase JSON field names.

* lib/model: Get rid of ConnectionInfo.MarshalJSON().

It was missing the StartedAt field from the embedded Statistics
struct.  Just lowercasing the JSON attribute names can be done just as
easily with annotations.

* lib/model: Remove bogus startedAt field from totals.

Instead of using the Statistics type with one field empty, just switch
to a free-form map with the three needed fields.

lib/protocol/protocol.go

@@ -994,10 +994,10 @@ func (c *rawConnection) pingReceiver() {
 }
 
 type Statistics struct {
-	At            time.Time
-	InBytesTotal  int64
-	OutBytesTotal int64
-	StartedAt     time.Time
+	At            time.Time `json:"at"`
+	InBytesTotal  int64     `json:"inBytesTotal"`
+	OutBytesTotal int64     `json:"outBytesTotal"`
+	StartedAt     time.Time `json:"startedAt"`
 }
 
 func (c *rawConnection) Statistics() Statistics {