Release TLS read and write buffers when idle

By default OpenSSL uses static large read/write buffers with TLS
connections. For memcached instances with a lot of client connections
this can quickly add up to gigabytes of memory. This options allows the
buffers to release when the clients are idle.

tls.c

@@ -228,6 +228,9 @@ int ssl_init(void) {
     SSL_CTX_set_options(settings.ssl_ctx, SSL_OP_NO_RENEGOTIATION);
 #endif
 
+    // Release TLS read/write buffers of idle connections
+    SSL_CTX_set_mode(settings.ssl_ctx, SSL_MODE_RELEASE_BUFFERS);
+
     return 0;
 }