Reduce the size of the pipe (#49270)

- Use the pipe itself as the synchronization object
- Store the options instance as a way to reference shared settings
- Added a field to PipeOptions for storing if the Pool is the ArrayPool implementation of the MemoryPool
- Shrink PipeAwaitable in the common case
  - Move the ExecutionContext and SynchronizationContext into a typed called the SchedulingContext. These types are mostly used with async await and it's extremely rare to have to capture any of this state.
- Shrink the size of PipeCompletion
  - Since completion callbacks are deprecated they are rarely set. We remove the pool and the other fields and just store a list (which should be rarely used now).
- Reduce the default segment pool size to 4 items = 16K buffered
  - The original size was optimized to avoid pool resizes but we need to balance idle memory and the potential resize cost of the resize.

src/libraries/System.IO.Pipelines/src/System.IO.Pipelines.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs"
-             Link="Common\System\Threading\Tasks\TaskToApm.cs" />
+         Link="Common\System\Threading\Tasks\TaskToApm.cs" />
     <Compile Include="Properties\InternalsVisibleTo.cs" />
     <Compile Include="System\IO\Pipelines\BufferSegment.cs" />
     <Compile Include="System\IO\Pipelines\CompletionData.cs" />
@@ -48,7 +48,7 @@
     <Compile Include="System\IO\Pipelines\ThreadPoolScheduler.netstandard.cs" />
     <Compile Include="System\IO\Pipelines\CancellationTokenExtensions.netstandard.cs" />
   </ItemGroup>
-   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or
                          $(TargetFramework.StartsWith('net4'))">
     <PackageReference Include="System.Buffers" Version="$(SystemBuffersVersion)" />
     <PackageReference Include="System.Memory" Version="$(SystemMemoryVersion)" />