journal: keep per-JournalFile location info during iteration

journal: keep per-JournalFile location info during iteration

In next_beyond_location() when we find a candidate entry in a journal
file, save its location information in struct JournalFile.

The purpose of remembering the locations of candidate entries is to be
able to save work in the next iteration. This patch does only the
remembering part.

LOCATION_SEEK means the location identifies a candidate entry.
When a winner is picked from among candidates, it becomes
LOCATION_DISCRETE.
LOCATION_TAIL here signifies we've iterated the file to the end (or the
beginning in the case of reversed direction).