Clean up packages and use internal object pool for stringbuilder

[manigandham]

• Cleanup unused nuget references
• Replace Microsoft.Extensions.ObjectPool with internal implementation using ConcurrentBag<> to avoid conflicts in some environments.

[manigandham]

No change in performance and all tests pass:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19043
AMD Ryzen 9 3900X, 1 CPU, 24 logical and 12 physical cores
.NET Core SDK=5.0.301
  [Host]     : .NET Core 5.0.7 (CoreCLR 5.0.721.25508, CoreFX 5.0.721.25508), X64 RyuJIT
  DefaultJob : .NET Core 5.0.7 (CoreCLR 5.0.721.25508, CoreFX 5.0.721.25508), X64 RyuJIT


|         Method |     Mean |     Error |    StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|--------------- |---------:|----------:|----------:|-------:|------:|------:|----------:|
|  RoundtripInts | 4.134 us | 0.0501 us | 0.0468 us | 0.0687 |     - |     - |     576 B |
| RoundtripLongs | 4.646 us | 0.0313 us | 0.0292 us | 0.0687 |     - |     - |     576 B |
|   RoundtripHex | 4.167 us | 0.0608 us | 0.0569 us | 0.1755 |     - |     - |    1512 B |