Add an ArrayPool<byte> from the System.Buffers namespace. With it, one
can exclude the System.ServiceModel.Primitives package.

In an effort to use NetMQ with Unity3D, one problem was trying to
satisfy the dependencies for `System.ServiceModel.Primitives`. It
required also providing `System.ServiceModel.{Http,Duplex,NetTcp}` and
perhaps more before I gave up. These dependencies were only noticed
while Unity3D built the project. When using NetMQ in a regular dotnet
project, I've had no problems with the `System.ServiceModel.Primitives`
dependencies.

I added an alternative implementation of `IBufferPool` using the
`ArrayPool` from `System.Buffers` but it is unused. I instrumented the
build system to have a "Flavor" that by default is set to "Standard",
which changes nothing from before.

For my usecase, however, I set the flavor to "Minimal".

    $ dotnet build -p:Flavor=Minimal

This excludes "System.ServiceModel.Primitives" and defines a
preprocessor flag of `FLAVOR_MINIMAL` that excludes the
BufferManagerBufferPool class and uses the ArrayBufferPool instead.

Subsitute with reference to System.Buffers.

I expect the BufferMangerBufferPool implementation will go away
but before that happens I wanted the tests for each implementation to
remain accessible. If you define USE_SERVICE_MODEL, it will build the
old pool implementation and use it. And it'll also run tests against the
old and new pool.