file frequency for med_phases_history_write is not correct inside fast coupling loop

Using ``med_phases_history_write`` in the fast coupling loop to write all inst fields in a single file does not create the correct time increment files.

For the following run sequence

```
runSeq::
@2400
MED med_phases_prep_ocn_avg
MED -> OCN :remapMethod=redist
OCN
@600
MED med_phases_prep_atm
MED med_phases_prep_ice
MED -> ATM :remapMethod=redist
MED -> ICE :remapMethod=redist
ATM
ICE
ATM -> MED :remapMethod=redist
MED med_phases_post_atm
ICE -> MED :remapMethod=redist
MED med_phases_post_ice
MED med_phases_prep_ocn_accum
MED med_phases_history_write
@
OCN -> MED :remapMethod=redist
MED med_phases_post_ocn
@
::
```

mediator history files are written correctly until the point when the next file would increment past the slow couple time (in this case, 2400s). For example, the next file in the sequence

```
ufs.cpld.cpl.hi.2021-03-22-24000.nc
ufs.cpld.cpl.hi.2021-03-22-24600.nc
```
should be at 25200 seconds (+600s), but the file created is at 27000 secs (+2400s ). Subsequent files are then created at the slow couple loop frequency.

If ``med_phases_history_write`` in placed in the slow loop in the above run sequence, all files are created at the right increments.