Since Provider instances do not know the index or type for which they are responsible, it's helpful for the ProviderRegistry to include that information when returning multiple providers.
* 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.
This introduces a registry service for persistence providers.
Previously, tagging one or more provider services would cause AddProviderPass to clobber the default providers created by the bundle's extension class. Now, the extension class tags its created providers and allows them to be registered via RegisterProvidersPass just like custom providers.
BC break: Custom providers tagged "foq_elastica.provider" must now specify a "type" attribute on their tag. An "index" attribute is optional (the default ES index will be used by default).
