Do not use entity_load*() methods

Thanks a lot for your work here, this already looks excellent, and you also seem to have caught all of them!

Just a few minor details:

1. +++ b/search_api_db/src/Plugin/search_api/backend/Database.php
   @@ -2034,11 +2034,6 @@ class Database extends BackendPluginBase {
   -    // Make the input lowercase as the indexed data is (usually) also all
   -    // lowercase.
   -    $incomplete_key = Unicode::strtolower($incomplete_key);
   -    $user_input = Unicode::strtolower($user_input);
   -
   

   That seems to be left over from an earlier patch?

2. +++ b/src/Plugin/search_api/datasource/ContentEntity.php
   @@ -523,8 +523,9 @@ class ContentEntity extends DatasourcePluginBase {
   +    $index_storage = $this->getEntityManager()->getStorage($this->getEntityTypeId());
   

   There is actually already $this->getEntityStorage() for that in this class.

3. +++ b/src/Tests/IntegrationTest.php
   @@ -34,6 +34,21 @@ class IntegrationTest extends WebTestBase {
   +  protected $index_storage;
   

   Now this is just my pedantic nit-picking about coding standards, but property names should actually be camelCase, not snake_case.
   But good idea to separate this into its own property instead of having that code in numerous places.

4. +++ b/src/Tests/IntegrationTest.php
   @@ -172,7 +188,8 @@ class IntegrationTest extends WebTestBase {
   +    $this->index_storage->resetCache([$index2_id]);
   

   Also coding style nit-picking, but even more specific: we're currently not using the shorthand array syntax in the Search API, and I'd like to remain consistent in that aspect.

Attached is an updated patch with those few problems removed. Also, while we're at it, I also went ahead and removed all the other calls to entity_*() functions (which are now all deprecated, it seems).
Sorry, didn't notice that earlier.

Forgot to remove the left-over database backend change in the previous patch.

That's the kind of stuff that happens if you try to be clever at midnight …

Great job, thanks! And you even changed the property name I forgot to align with the coding standards, nice.
So, committed.
Thanks a lot again!