Public API cleanup

[marcinbudny]

Description

This pull requests aims to clean public API surface before 1.0 release. List of changes:

Item	Change
Proto.ActorSystem.Stopper	Made internal. Use ShutdownAsync to stop actor system, StoppedReason to retrieve reason for shutdown
Proto.ActorSystem.ConfigureProps, ConfigureSystemProps	Made internal.
Proto.IContext.Stash()	Related functionality removed. Use IContext.Capture and CapturedContext.Receive
Proto.PID.SendSystemMessage()	Made internal.
Proto.Channels.ChannelPublisher, ChannelSubscriber	Removed and converted to an example.
Proto.MessageBatch	Removed.
Proto.Utils.TypeDictionary	Made internal
Proto.ActorContextExtras.Store	Made internal. Use Proto.ActorContext.Get, Set, Remove
Proto.Remote.GrpcNet.Extensions.AddRemote, AddClientRemote, UseProtoRemote	Made internal.
Proto.Cluster.ClusterConfig.TimeoutTimespan	Made obsolete. Use ActorActivationTimeout instead. The TimeoutTimespan implementation is now a proxy for ActorActivationTimeout
Proto.Cluster.ClusterConfig.ActorSpawnTimeout	Renamed to ActorSpawnVerificationTimeout
Proto.Cluster.Cluster.Logger, PidCache, TryGetClusterKind	Made internal.
Proto.Cluster.MemberList.TopologyConsensus, GetActivator, GetMetaMember, TryGetMemberIndexByAddress, TryGetMemberByIndex	Made internal.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[mhelleborg]

LGTM 😄