Fix race in entity_platform.async_add_entities

[emontnemery]

Description:

There is a race in entity_platform.async_add_entities() which makes the check for duplicated entity_id fail. This in turns may cause several entities to be created with the same entity_id.
The issue can very easily be reproduced by publishing persistent MQTT discovery messages with same name before starting hass.

This has previously been reported in #13316 and #15731, although with a slightly different twist
The root cause is described in some detail in #13316

This PR solves the race when there are multiple concurrent calls to entity_platform.async_add_entities() from the same platform.

Possible further improvement:

• Also detect multiple multiple concurrent calls to entity_platform.async_add_entities() from different platforms as pointed out by @OttoWinter
  Maybe this can be fixed by:
  • Add a lock to not allow concurrent calls to entity_platform.async_add_entities() - dead simple, but might have a negative performance impact
  • Add a dictionary reserved_entity_ids which will hold an entity_id until the entity has been added to hass.states
  • Format the entity_id generated by entity_registry.async_generate_entity_id to also include platform name, e.g. mqtt_name instead of name in the same way as is already done by homeassistant.helpers.entity.async_generate_entity_id

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

[balloob]

Nice fix! I think that we should make a better "ensure_unique_string" as there is no reason to chain these things, if anything we should use a ChainMap.