SubscribeMulti + UnsubscribeMulti, multiplicities (#249)

## Description of Changes
Add SubscribeMulti and UnsubscribeMulti from upstream.
Fix unsubscribe bug found by testing against Bitcraft: Calling
`UnsubscribeThen` with any (non-null) callback would result in an
exception incorrectly telling the user that `Unsubscribe` had been
called twice.
Multiplicity support is implemented with unit tests + manual testing of
quickstart-chat.
Also, rows in the client cache are now looked up by primary key if
available, which I suspect is going to be a large performance boost.

## API

 - [x] This is an API breaking change to the SDK

*If the API is breaking, please state below what will break*

## Requires SpacetimeDB PRs
*List any PRs here that are required for this SDK change to work*

## Testsuite
*If you would like to run the your SDK changes in this PR against a
specific SpacetimeDB branch, specify that here. This can be a branch
name or a link to a PR.*

SpacetimeDB branch name: jgilles/final-cs-codegen-changes

## Testing
*Write instructions for a test that you performed for this PR*

- [x] unit tests
- [ ] Working with Ryan to add a test suite.
- [x] blackholio

Author: kazimuth

README.md

@@ -2,7 +2,7 @@
 
 ## Overview
 
-This repository contains the [Unity](https://unity.com/) SDK for SpacetimeDB. The SDK allows to interact with the database server and is prepared to work with code generated from a SpacetimeDB backend code.
+This repository contains the C# and [Unity](https://unity.com/) SDKs for SpacetimeDB. The SDK allows to interact with the database server and is prepared to work with code generated from a SpacetimeDB backend code.
 
 ## Documentation
 

examples~/quickstart/client/Program.cs

@@ -94,6 +94,7 @@ void PrintMessage(RemoteTables tables, Message message)
 
 void Message_OnInsert(EventContext ctx, Message insertedValue)
 {
+
     if (ctx.Event is not Event<Reducer>.SubscribeApplied)
     {
         PrintMessage(ctx.Db, insertedValue);
@@ -123,25 +124,15 @@ void OnConnect(DbConnection conn, Identity identity, string authToken)
     local_identity = identity;
     AuthToken.SaveToken(authToken);
 
-    var subscriptions = 0;
-    Action<SubscriptionEventContext> waitForSubscriptions = (SubscriptionEventContext ctx) =>
-    {
-        // Note: callbacks are always invoked on the main thread, so you don't need to
-        // worry about thread synchronization or anything like that.
-        subscriptions += 1;
-
-        if (subscriptions == 2)
-        {
-            OnSubscriptionApplied(ctx);
-        }
-    };
-
-    var userSubscription = conn.SubscriptionBuilder()
-        .OnApplied(waitForSubscriptions)
-        .Subscribe("SELECT * FROM user");
-    var messageSubscription = conn.SubscriptionBuilder()
-        .OnApplied(waitForSubscriptions)
-        .Subscribe("SELECT * FROM message");
+    var subscription = conn.SubscriptionBuilder()
+        .OnApplied(OnSubscriptionApplied)
+        .Subscribe(new string[] {
+            "SELECT * FROM user",
+            "SELECT * FROM message",
+            // It is legal to have redundant subscriptions.
+            // However, keep in mind that data will be sent over the wire multiple times,
+            // once for each subscriptions. This can cause slowdowns if you aren't careful.
+            "SELECT * FROM message" });
 
     // You can also use SubscribeToAllTables, but it should be avoided if you have any large tables:
     // conn.SubscriptionBuilder().OnApplied(OnSubscriptionApplied).SubscribeToAllTables();