ArrayBuffer tweaks

[japasc]

I cant create a PR, sorry.
Two observations

• In method ConfigurableArrayPool.Return

if (clearArray)
                {
                    Array.Clear(array);
                }

should be outside

            if (haveBucket)
            {

otherwise memory goes unclean and unnoticed.

• In method Bucket.Rent
  allocateBuffer = buffer == null;
  is unnecessay inside the spin.
  you can remove the variable allocatedBuffer
  and change if (allocateBuffer) => if (buffer == null)
  as buffer is a local variable

[teo-tsirpanis]

• If haveBucket is false the array will be eligible for collection by the GC. It doesn't matter if it holds references to other objects, the GC will be able to collect them too.
• You are likely right about this one but the difference would be so tiny I doubt it would be even detected by benchmarks.

[PathogenDavid]

It's also worth noting that the clear behavior is explicitly documented:

If clearArray is set to true, and if the pool will store the buffer to enable subsequent reuse, the Return(T[], Boolean) method will clear the array [...]