Add the option to report WAL usage in EXPLAIN and auto_explain.

Add the option to report WAL usage in EXPLAIN and auto_explain.

This commit adds a new option WAL similar to existing option BUFFERS in the
EXPLAIN command.  This option allows to include information on WAL record
generation added by commit df3b181499 in EXPLAIN output.

This also allows the WAL usage information to be displayed via
the auto_explain module.  A new parameter auto_explain.log_wal controls
whether WAL usage statistics are printed when an execution plan is logged.
This parameter has no effect unless auto_explain.log_analyze is enabled.

Author: Julien Rouhaud
Reviewed-by: Dilip Kumar and Amit Kapila
Discussion: https://postgr.es/m/CAB-hujrP8ZfUkvL5OYETipQwA=e3n7oqHFU=4ZLxWS_Cza3kQQ@mail.gmail.com