rename SnapBuild* macros in slot.c

Hi,

It seems like the same macro names for
SnapBuildOnDiskNotChecksummedSize and SnapBuildOnDiskChecksummedSize
are being used in slot.c and snapbuild.c. I think, in slot.c, we can
rename them to ReplicationSlotOnDiskNotChecksummedSize and
ReplicationSlotOnDiskChecksummedSize
similar to the other macros ReplicationSlotOnDiskConstantSize and
ReplicationSlotOnDiskV2Size.

Here's a tiny patch for the above change.

Regards,
Bharath Rupireddy.