Add FLUSH_UNLOGGED option to CHECKPOINT command.

Add FLUSH_UNLOGGED option to CHECKPOINT command.

This option, which is disabled by default, can be used to request
the checkpoint also flush dirty buffers of unlogged relations.  As
with the MODE option, the server may consolidate the options for
concurrently requested checkpoints.  For example, if one session
uses (FLUSH_UNLOGGED FALSE) and another uses (FLUSH_UNLOGGED TRUE),
the server may perform one checkpoint with FLUSH_UNLOGGED enabled.

Author: Christoph Berg <[email protected]>
Reviewed-by: Laurenz Albe <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Reviewed-by: Dilip Kumar <[email protected]>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de