Include information on buffer usage during planning phase, in EXPLAIN output, take...

 Include information on buffer usage during planning phase, in EXPLAIN output, take two.
 
 When BUFFERS option is enabled, EXPLAIN command includes the information
 on buffer usage during each plan node, in its output. In addition to that,
 this commit makes EXPLAIN command include also the information on
 buffer usage during planning phase, in its output. This feature makes it
 easier to discern the cases where lots of buffer access happen during
 planning.
 
 This commit revives the original commit ed7a509571 that was reverted by
 commit 19db23bcbd. The original commit had to be reverted because
 it caused the regression test failure on the buildfarm members prion and
 dory. But since commit c0885c4c30 got rid of the caues of the test failure,
 the original commit can be safely introduced again.
 
 Author: Julien Rouhaud, slightly revised by Fujii Masao
 Reviewed-by: Justin Pryzby
 Discussion: https://postgr.es/m/16109-26a1a88651e90608@postgresql.org