Remove task allocation in unary call handler

[JamesNK]

Before

          Method |     Mean |     Error |    StdDev |      Op/s | Gen 0/1k Op | Gen 1/1k Op | Gen 2/1k Op | Allocated Memory/Op |
---------------- |---------:|----------:|----------:|----------:|------------:|------------:|------------:|--------------------:|
 HandleCallAsync | 3.395 us | 0.0629 us | 0.0589 us | 294,547.7 |      0.0191 |           - |           - |             1.42 KB |

After

          Method |     Mean |     Error |    StdDev |      Op/s | Gen 0/1k Op | Gen 1/1k Op | Gen 2/1k Op | Allocated Memory/Op |
---------------- |---------:|----------:|----------:|----------:|------------:|------------:|------------:|--------------------:|
 HandleCallAsync | 3.228 us | 0.0390 us | 0.0345 us | 309,742.3 |      0.0153 |           - |           - |             1.35 KB |