Fix a crash by avoiding destroying TLS stream socket too early

This commit fixes a crash in generic TLS stream code, which could be
reproduced during some runs of the 'sslyze' tool.

The intention of this commit is twofold.

Firstly, it ensures that the TLS socket object cannot be destroyed too
early. Now it is being deleted alongside the underlying TCP socket
object.

Secondly, it ensures that the TLS socket object cannot be destroyed as
a result of calling 'tls_do_bio()' (the primary function which
performs encryption/decryption during the IO) as the code did not
expect that. This code path is fixed now.

lib/isc/netmgr/netmgr-int.h

@@ -966,6 +966,7 @@ struct isc_nmsocket {
 						    worker */
 		size_t n_listener_tls_ctx;
 		isc_nmsocket_t *tlslistener;
+		isc_nmsocket_t *tlssocket;
 		atomic_bool result_updated;
 		enum {
 			TLS_INIT,