firebase-ios-sdk

Merge persistence-master to master (#3829)

* Port FSTMemoryPersistence to C++ (#3754)

* Add local::Persistence

* Add MemoryPersistence

* Add MemoryEagerReferenceDelegate

* Make LruDelegate extend ReferenceDelegate

This enables downcasting to LruDelegate in tests that need to do so.

* Add MemoryLruReferenceDelegate

* Don't double-count document key size.

* Port FSTLevelDB to C++ (#3755)

* Add LevelDbPersistence

Note that Apple-specific initialization has been partitioned off in
anticipation of being able to run this code on non-Apple platforms.

* Add LevelDbLruReferenceDelegate

* Migrate to C++ Persistence (1/2) (#3756)

* Use C++ Persistence classes in FSTLocalStore

* Remove ReferenceDelegateBridge

* Cut src over to C++ persistence types

* Remove Objective-C requirement from some classes

... that no longer require it.

* Migrate to C++ Persistence (2/2) (#3757)

* Remove LruDelegateBridge

* Update test initialization of persistence components

In particular, ensure that there's always an owner for the Persistence
instance, which wasn't always handled explicitly in the Objective-C
version of the tests.

* Largely mechanical changes to tests

* Remove Objective-C persistence classes

* Forward declare util::Status when possible (#3761)

* Add a header dedicated to forward declarations for util::Status

* Use status_fwd.h wherever possible