Add option list to CHECKPOINT command.

Add option list to CHECKPOINT command.

This commit adds the boilerplate code for supporting a list of
options in CHECKPOINT commands.  No actual options are supported
yet, but follow-up commits will add support for MODE and
FLUSH_UNLOGGED.  While at it, this commit refactors the code for
executing CHECKPOINT commands to its own function since it's about
to become significantly larger.

Author: Christoph Berg <myon@debian.org>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de