checkpointer: Request checkpoint via latch instead of signal

checkpointer: Request checkpoint via latch instead of signal

The motivation for this change is that a future commit will use SIGINT for
another purpose (postmaster requesting WAL access to be shut down) and that
there no other signals that we could readily use (see code comment for the
reason why SIGTERM shouldn't be used). But it's also a tad nicer / more
efficient to use SetLatch(), as it avoids sending signals when checkpointer
already is busy.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu@m3cfzxicm5kp