Inserting and Deleting a Relation in the same Transaction fails

[flyingsilverfin]

Description

Inserting and deleting a relation in the same transaction throws an error such as:
UNKNOWN: Vertex with id 4344 was removed..

Environment

Grakn Core 1.5 (any current master should fail)

Reproducible Steps

In console:

1. define person sub entity, plays friend; friendship sub relation, relates friend;
2. commit
3. insert $x isa person; $y isa person; $r (friend: $x, friend: $y) isa friendship;
4. collect ID of $r above -- match $r id <R ID>; delete $r;
5. commit

Expected Output

Should insert $x and $y but not the friendship

Actual Output

UNKNOWN: Vertex with id 4344 was removed..

Additional information

Investigation pins this down to the two implementations of relation - Relation and RelationReified. These interact badly with the TransactionCache... the inserted friendship goes into TransactionCache.newRelations, but when deleting $r, this is a RelationReified and does not remove concepts from newRelations.

A proper fix would fix the concept hierarchy and where RelationReified fits into it.

Flagged up by #4447 and #5084

[flyingsilverfin]

@kasper-piskorski