If a user configures query_builder_method for the doctrine provider, and
does not alias the root entity to equal Provider::ENTITY_ALIAS then a
DQL error will occur during index population.
> [Semantical Error] [...] near 'a.id ASC': Error: 'a' is not defined.
Provider::countObjects already handles this by dynamically fetching the
alias with QueryBuilder::getRootAliases, and Provider::fetchSlice should
be doing the same.
nb. "Provider" refers to FOS\ElasticaBundle\Doctrine\ORM\Provider
* Created AbstractProvider class (for all DB services), which handles the default batch_size option.
* The logger Closure is now optional for populate().
* Removed unused Elastica_Type argument from Provider constructors.
* Added unit tests for Doctrine's AbstractProvider class.
* The extra argument (ManagerRegistry) for Doctrine providers is now an appended constructor argument, so the extension no longer needs to use different replacement indexes for Propel/Doctrine providers.
