Preliminary fix for message timing with cli_progress_along() (#668)

[rundel]

The underlying issue seems to be that message printing was just looking at show_after value which is fixed. I believe the change below is the desired behavior where we compare now with show_after + last_shown instead.

The code is a bit convoluted because we need to grab last_shown which does not necessarily exist. If it is not yet defined then I've defaulted to -Inf so that the first iteration will always print regardless of show_after.

Fix #668

[rundel]

It is also very likely that I am missing necessary PROTECT() and UNPROTECT() calls - it's been way too long since I've worked directly with the C interface.