postgres_fdw: Fix connection leak.

 postgres_fdw: Fix connection leak.
 
 In postgres_fdw, the cached connections to foreign servers will not be
 closed until the local session exits if the user mappings or foreign servers
 that those connections depend on are dropped. Those connections can be
 leaked.
 
 To fix that connection leak issue, after a change to a pg_foreign_server
 or pg_user_mapping catalog entry, this commit makes postgres_fdw close
 the connections depending on that entry immediately if current
 transaction has not used those connections yet. Otherwise, mark those
 connections as invalid and then close them at the end of current transaction,
 since they cannot be closed in the midst of the transaction using them.
 Closed connections will be remade at the next opportunity if necessary.
 
 Back-patch to all supported branches.
 
 Author: Bharath Rupireddy
 Reviewed-by: Zhihong Yu, Zhijie Hou, Fujii Masao
 Discussion: https://postgr.es/m/CALj2ACVNcGH_6qLY-4_tXz8JLvA+4yeBThRfxMz7Oxbk1aHcpQ@mail.gmail.com