DTLS: Use bio callback to get fragment packet. v5.0.156, v6.0.47 (oss…

…rs#3565)

1. The MTU is effective, with the certificate being split into two DTLS records to comply with the limit.
2. The issue occurs when using BIO_get_mem_data, which retrieves all DTLS packets in a single call, even though each is smaller than the MTU.
3. An alternative callback is available for using BIO_new with BIO_s_mem.
4. Improvements to the MTU setting were made, including adding the DTLS_set_link_mtu function and removing the SSL_set_max_send_fragment function.
5. The handshake process was refined, calling SSL_do_handshake only after ICE completion, and using SSL_read to handle handshake messages.
6. The session close code was improved to enable immediate closure upon receiving an SSL CloseNotify or fatal message.

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Co-authored-by: chundonglinlin <chundonglinlin@163.com>

trunk/src/app/srs_app_rtc_conn.cpp

@@ -3860,7 +3860,8 @@ srs_error_t SrsRtcConnection::on_dtls_alert(std::string type, std::string desc)
     srs_error_t err = srs_success;
 
     // CN(Close Notify) is sent when client close the PeerConnection.
-    if (type == "warning" && desc == "CN") {
+    // fatal, IP(Illegal Parameter) is sent when DTLS failed.
+    if (type == "fatal" || (type == "warning" && desc == "CN")) {
         SrsContextRestore(_srs_context->get_id());
         switch_to_context();