Add support for performing writes using legacy serialization

[Aaronontheweb]

Version: Akka.Persistence.MongoDb v1.4.0-beta2

Proposal is to add a feature flag to allow users who've been using the legacy BSON serialization for MongoDb to continue doing so despite #72 (comment)

The primary use case this is intended to address is the legacy data issue:

What am I supposed to do with all of my historically saved data in MongoDB?

We're going to work on that and see if it's possible, through the use of Akka.Peristence EventAdapters or something along those lines to provide forward compatibility for older events. Ideally, we would like to not have anyone stranded on an old version of Akka.Persistence and this is a problem we're going to have to solve across multiple plugins.

Reads in Akka.Persistence.MongoDb are already forwards compatible, so that's not a problem.

However, writes automatically default to protobuf serialization right now - because that's the only way to feasibly support issues like IActorRef serialization for Akka.Cluster.Sharding and so on.

We'll add a feature flag to the MongoDb driver along the lines of:

akka.persistence.mongodb{
   journal{
    legacy-serialization = off
  }

  snapshot-store{
   legacy-serialization = off
 }
}

If you opt into using this feature, you're acknowledging that "yes, I know Akka.Cluster.Sharding, AtLeastOnceDeliveryActors, and so on may not work properly - I am ok with that."

[Aaronontheweb]

Working on an implementation for this now.