MINOR: server: enable slowstart for dynamic server

Enable the 'slowstart' keyword for dynamic servers. The slowstart task
is allocated in 'add server' handler if slowstart is used.

As the server is created in disabled state, there is no need to start
the task. The slowstart task will be automatically started on the first
'enable server' invocation.
diff --git a/doc/management.txt b/doc/management.txt
index 64172e5..f0c28a4 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1548,6 +1548,7 @@
   - send-proxy-v2
   - send-proxy-v2-ssl
   - send-proxy-v2-ssl-cn
+  - slowstart
   - sni
   - source
   - ssl