Add custom async channel pool

[ArieGato]

At the moment the package Microsoft.Extensions.ObjectPool is used for managing a pool of channels/connections. The object pool is not async, so creating a channel in the object pool is done with an AsyncHelper that creates a custom SynchronizationContext.

Creating a custom channel pool will resolve this issue. In doing so a package dependency is also removed from the package.

[ArieGato]

@sungam3r Do you have any ideas how we can solve this? I have a branch, but I'm not satisfied about the solution.

[sungam3r]

I don't know. I haven't looked into the code yet. Essentially I'll be watching it tomorrow.

[sungam3r]

The first thought so far - why to use that pool at all? Why not to use single channel? Performance? Throughput? What if preallocate N channels at the start and not pool them? Pool is essentially for lightweight objects, not ones that participate in long-running async I/O operations.

[ArieGato]

a channel is not thread safe, so it cannot be used my multiple threads simultaneously. So I think a pool is a good solution.

[sungam3r]

Pool is about object reuse, not about its initialization, especially async. You may just init all channels beforehand and store them - in pool or other structure.

[ArieGato]

It's an interesting approach. I am a bit reluctant to initialize all channels due to resource usage. Then again, when the application logs intensively, the resources will be allocated anyway.