CNS-496: fixation allow append after delete

[orenl-lava]

Changed fixation to allow AppendEntry() after DelEntry() takes effect, with the following new rules:

• AppendEntry() may add retroactive entry only if it does not precede an existing latest entry.
• AppendEntry() may not add future entries on or beyond a DeletedAt in the future.
• Adding a future entry maturing on a future DeleteAt is permitted but useless, because it will be deleted straight away).
• DelEntry() discards all future entries on or beyond the DeletedAt (and owing to the previous rule, none will be added until thy delete actually takes place).

Also fix a few glitches in the previous code:

• The check for retroactive invalid Append was off-by-one.
• If the first entry appended was future - the code did not set a time for its maturity block (so it never matures)
• Deleted entries incorrectly retained their IsLatest flag
• Entries with DeleteAt superseded by maturing future entries incorrectly retained their DeleteAt
• Fixation testing advanced multiple block without going calling AdvanceBlock() for each block