Fix inconsistent elements after server restart

Elements for the Pool were cached on the first read.
If a client is reading this attribute during server startup, the list returned might be incomplete.
As it's cached, it will remain incomplete.
This can easily happen if the Pool is restarted while the MacroServer is still running,
or another GUI is polling Elements.

The value should only be cached when the server is running to avoid this issue.

Initial issue was for the Pool, but same thing applies for the MacroServer.

Fix #113