Deprecate calling commands on the original Redis instance in multi {}

The new favored API is:

```ruby
redis.multi do |transaction|
  transaction.get("foo")
  transaction.del("bar")
end
```

This API allow multiple threads to build pipelines concurrently on the same
connection, and is more friendly to Fiber based concurrency.

Fix: redis#1057

CHANGELOG.md

@@ -15,6 +15,20 @@
     pipeline.get("key")
   end
   ```
+* Deprecate calling commands on `Redis` inside `Redis#multi`. See #1059.
+  ```ruby
+  redis.multi do
+    redis.get("key")
+  end
+  ```
+
+  should be replaced by:
+
+  ```ruby
+  redis.multi do |transaction|
+    transaction.get("key")
+  end
+  ```
 * Deprecate `Redis#queue` and `Redis#commit`. See #1059.
 
 * Fix `zpopmax` and `zpopmin` when called inside a pipeline. See #1055.

lib/redis.rb

@@ -216,7 +216,7 @@ def unwatch
   def pipelined(&block)
     deprecation_displayed = false
     if block&.arity == 0
-      Pipeline.deprecation_warning(Kernel.caller_locations(1, 5))
+      Pipeline.deprecation_warning("pipelined", Kernel.caller_locations(1, 5))
       deprecation_displayed = true
     end
 
@@ -263,19 +263,27 @@ def pipelined(&block)
   #
   # @see #watch
   # @see #unwatch
-  def multi
-    synchronize do |prior_client|
-      if !block_given?
-        prior_client.call([:multi])
-      else
+  def multi(&block)
+    if block_given?
+      deprecation_displayed = false
+      if block&.arity == 0
+        Pipeline.deprecation_warning("pipelined", Kernel.caller_locations(1, 5))
+        deprecation_displayed = true
+      end
+
+      synchronize do |prior_client|
         begin
-          @client = Pipeline::Multi.new(prior_client)
-          yield(self)
-          prior_client.call_pipeline(@client)
+          pipeline = Pipeline::Multi.new(prior_client)
+          @client = deprecation_displayed ? pipeline : DeprecatedPipeline::Multi.new(pipeline)
+          pipelined_connection = PipelinedConnection.new(pipeline)
+          yield pipelined_connection
+          prior_client.call_pipeline(pipeline)
         ensure
           @client = prior_client
         end
       end
+    else
+      send_command([:multi])
     end
   end
 

lib/redis/pipeline.rb

@@ -44,19 +44,19 @@ class Pipeline
     STDLIB_PATH = File.expand_path("..", MonitorMixin.instance_method(:synchronize).source_location.first).freeze
 
     class << self
-      def deprecation_warning(caller_locations) # :nodoc:
+      def deprecation_warning(method, caller_locations) # :nodoc:
         callsite = caller_locations.find { |l| !l.path.start_with?(REDIS_INTERNAL_PATH, STDLIB_PATH) }
         callsite ||= caller_locations.last # The caller_locations should be large enough, but just in case.
         ::Redis.deprecate! <<~MESSAGE
           Pipelining commands on a Redis instance is deprecated and will be removed in Redis 5.0.0.
 
-          redis.pipelined do
+          redis.#{method} do
             redis.get("key")
           end
 
           should be replaced by
 
-          redis.pipelined do |pipeline|
+          redis.#{method} do |pipeline|
             pipeline.get("key")
           end
 
@@ -195,7 +195,22 @@ def initialize(pipeline)
 
     def __getobj__
       unless @deprecation_displayed
-        Pipeline.deprecation_warning(Kernel.caller_locations(1, 10))
+        Pipeline.deprecation_warning("pipelined", Kernel.caller_locations(1, 10))
+        @deprecation_displayed = true
+      end
+      @delegate_dc_obj
+    end
+  end
+
+  DeprecatedPipeline::Multi = DelegateClass(Pipeline::Multi) do
+    def initialize(pipeline)
+      super(pipeline)
+      @deprecation_displayed = false
+    end
+
+    def __getobj__
+      unless @deprecation_displayed
+        Pipeline.deprecation_warning("multi", Kernel.caller_locations(1, 10))
         @deprecation_displayed = true
       end
       @delegate_dc_obj