Fix infinite loop when parsing invalid DAP message.

This commit updates `ContentReader::read` to optionally close the
underlying `Reader` when a message is read that does not terminate
the DAP header immediately following the `Content-Length` header
field.

This fixes an infinite loop when parsing an invalid DAP message. In
particular, the infinite loop occurs when a cross-site HTTP request from
a browser running on the same host as a cppdap server is sent to the
server. As cross-site requests to the localhost can be triggered by
untrusted JavaScript, more validation of the DAP messages is required to
protect cppdap-based servers from malicious requests.

This commit introduces a new enum flag on both `Session` and
`ContentReader` to allow a user to indicate that the underlying `Reader`
should be closed when an invalid DAP message is received. The
server example has been updated to indicate that invalid messages
should result in closure of the underlying `Reader`.

examples/simple_net_client_server.cpp

@@ -30,6 +30,11 @@ int main(int, char*[]) {
   auto onClientConnected =
       [&](const std::shared_ptr<dap::ReaderWriter>& socket) {
         auto session = dap::Session::create();
+
+        // Set the session to close on invalid data. This ensures that data received over the network
+        // receives a baseline level of validation before being processed.
+        session->setOnInvalidData(dap::kClose);
+
         session->bind(socket);
 
         // The Initialize request is the first message sent from the client and