[#341] epoch_interval setting in the journal file needs to be honored in all cases

[nars1]

Code fixes for GTM-8708 in GT.M V6.3-002 wanted to prevent the update helper writer process from
unnecessarily making calls to grab_crit_immediate() in case the instance had no updates.
It did this by setting the next epoch time (jbp->next_epoch_time) to a huge value (MAXUINT4)
the moment a wcs_flu() call was done to write a regular epoch (JRE gvstat) where it noticed an
idle epoch (JRI gvstat) had already been written (because of inactivity since the last update).
This caused the jbp->next_epoch_time vs jgbl.gbl_jrec_time check in updhelper_writer() to
automatically skip invoking the grab_crit_immediate() during periods of idleness.

But this introduced the #341 regression. It is now possible for a regular epoch to not be written
in a timely fashion. This is because once jbp->next_epoch_time gets set to MAXUINT4 as part of
an update, if the next update on the same journal file happens say N seconds later (where N could
be as high as 5 seconds, the hardcoded inactivity time before an idle epoch kicks in), the call
to t_end/tp_tend for the next update finds jbp->next_epoch_time set to MAXUINT4 and resets it to
the current time (which is the time of the next update, not the time of the prior update) plus
the epoch_interval. This means the next_epoch_time is set to a value that is incorrect by at most
N seconds. That is, it is possible for two updates to be separated by as much as N + epoch_interval
seconds without having an intervening EPOCH in the journal file. This violates the definition of
the epoch_interval setting.

The fix for this is to undo all the changes done as part of GTM-8708. So jbp->next_epoch_time is
never set to MAXUINT4. Instead, updhelper_writer.c is enhanced to check if an EPOCH is the most
recent record in the journal buffer (jbp->post_epoch_freeaddr == jbp->rsrv_freeaddr) and if so
we skip checking jbp->next_epoch_time (and invoking grab_crit_immediate()) altogether since we know
there is nothing left to be flushed in the journal file. This achieves the objective of GTM-8708.
And for #341, jbp->next_epoch_time is updated in wcs_flu() at the time when we notice an epoch is
the most recent journal record in the journal file. This ensures we honor the epoch_interval setting.