Since this change the client is responsible to provide credentials
to use to authenticate backends (through ESource-s, to be more precise),
unless the credentials are already saved.
Returns TRUE if an EClient instance for the given source and extension
name was recently discarded after having emitted a "backend-died" signal,
and a replacement EClient instance has not yet been created.
New class to help reduce code duplication and centralize some EClient
handling policies.
Benefits:
- EClient instances can be shared across the entire application.
- Centralized rebroadcasting of "backend-died" and "backend-error"
signals emitted from cached EClient instances.
- Automatic cache invalidation when backends crash. The EClient
is discarded, and a new instance is created on the next request.
