runtime: Performance of System.Console.CursorLeft and System.Console.CursorTop is really, really bad in .NET Core 2.1.2 on Linux

Test case:

I am trying to use this library as a progress bar for a netcoreapp2.1 console app on Linux. However, I found that since it uses System.Console.CursorLeft and System.Console.CursorTop frequently (in loops), that it was completely unusable. If you click on the link, there are some GIFs in the README that show the demo program completing in about 15-20 seconds. I let the demo program run on .NET Core 2.1.2 on Linux for 12 hours!!! and it was still on 0%. I did some profiling on this and found that 50% of the CPU time was in system calls to the kernel reading and writing stdin.

I’ve read all of the comments in https://github.com/dotnet/corefx/blob/master/src/System.Console/src/System/ConsolePal.Unix.cs, so I understand that there are always going to be performance issues with this on Unix platforms, but this seems so extreme that surely it can be improved somewhat.

I made this pull request to work around the issue, so hopefully it might help anyone else facing slowdowns because of this.

Steps to reproduce:

• git clone https://github.com/Mpdreamz/shellprogressbar.git
• git checkout 21a516004680 for netcoreapp1.1 or git checkout cd998d37d7f7 for netcoreapp2.1
• dotnet run -p src/ShellProgressBar.Example

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   2.1.302
 Commit:    9048955601

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  18.04
 OS Platform: Linux
 RID:         ubuntu.18.04-x64
 Base Path:   /usr/share/dotnet/sdk/2.1.302/

Host (useful for support):
  Version: 2.1.2
  Commit:  811c3ce6c0

.NET Core SDKs installed:
  2.1.302 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 1.1.9 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.2 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download