When CREATE SERVER is issued, it allocates memory on memory root
to store cached server structure. When DROP SERVER is issued,
it doesn't release this memory, as it is impossible with the
memory root.

We use the same allocation strategy for plugins and acl. The problem
here that there was no way (except for the server restart) to force
'servers' code to release this memory.

With this fix it is possible to release unused server cache memory
by FLUSH PRIVILEGES.

No test case for this fix.