Entity - Alias inconsistency when creating several alias in parallel

[jorgemarey]

Describe the bug
I was developing the identity resources in the vault terraform provider and found a problem when creating aliases to entities. If (like terraform does) several aliases are created at the same time, the entity alias is created correctly but the entity object doesn't reference all of the aliases (it doesn't get updated accordingly). I don't know if this translate into future problems, but I thought it was worth noticing.

To Reproduce

1. vault server -dev
2. vault auth list -format=json (get the accessor of any backend, for example token)
3. create an entity in vault :
   curl -H "X-Vault-Token: ${VAULT_TOKEN}" -k -X POST -d '{}' ${VAULT_ADDR}/v1/identity/entity and save the ID
4. lauch in parallel:

for i in $(seq 1 100)                                                                           
do
curl -H "X-Vault-Token: ${VAULT_TOKEN}" -k -X POST -d '{"mount_accessor": "*****ACCESSOR_HERE*****", "canonical_id": "*****ENTITY_ID_HERE*****", "name": "'"$i"'"}' ${VAULT_ADDR}/v1/identity/entity-alias
done

for i in $(seq 101 200)                                                                           
do
curl -H "X-Vault-Token: ${VAULT_TOKEN}" -k -X POST -d '{"mount_accessor": "*****ACCESSOR_HERE*****", "canonical_id": "*****ENTITY_ID_HERE*****", "name": "'"$i"'"}' ${VAULT_ADDR}/v1/identity/entity-alias
done

5. The entity created previously, if obtained, should have all aliases, but instead some are missing.

I know that 200 aliases are overkill, but I saw this behaviour happen even by creating only 2 aliases in parallel.

Expected behavior
All aliases are tied to the entity.

Environment:

$ vault version
Vault v0.10.3 ('c69ae68faf2bf7fc1d78e3ec62655696a07454c7')

Additional context
I was looking into the code and I think that this is related to that the entity is obtained here
https://github.com/hashicorp/vault/blob/master/vault/identity_store_aliases.go#L215
and saved there
https://github.com/hashicorp/vault/blob/master/vault/identity_store_aliases.go#L356
If another alias creation is started when the first one is not yet saved, the first will get the value from memdb whitout that value, and when upserting it, that value would be lost.

I'm happy to help by providing some more info if needed.