Fix duplicate entities

[steven-sheehy]

Detailed description:

• Changes unique index and lookup criteria for t_entities to be (entity_shard, entity_realm, entity_num)
• Migrates duplicate entities that have both account and contract to use the contract entity

Which issue(s) this PR fixes:
Fixes #251

Special notes for your reviewer:
The contract is always created first for these dupes, hence why min(id) chooses the contract entity in the migration

Checklist

• Documentation added
• Tests updated

[mike-burrage-hedera]

Looks good. rc2

[gregscullard]

The postgres function f_entity_create also needs updating

  SELECT id
  INTO entity_id
  FROM t_entities
  WHERE entity_shard = _shard
  AND   entity_realm = _realm
  AND   entity_num = _num
  AND   fk_entity_type_id = _type_id;

Suggest removal of last line of where clause (And fk_entity_type = ...).

It is defined in sql migrations v1.3__procedures.sql