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.


sql/sql_parse.cc:
  Reload servers table on FLUSH PRIVILEGES.
sql/sql_servers.cc:
  Instead of just marking memory blocks as unused, release memory
  used by servers cache and initialize new memory root.
  
  This is needed for FLUSH PRIVILEGES to release unused memory
  blocks.