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ARROW-4503: [C#] Eliminate allocations in ArrowStreamReader when reading from a Stream

When reading `RecordBatch` instances from a .NET `Stream` using the `ArrowStreamReader` class, it is currently allocating and copying memory 3 times for the data.

I've changed it to take a MemoryPool in the constructor, `ArrowStreamReader` will allocate from the pool a single time for each `RecordBatch` it reads, copy from the `Stream` into that memory once, and then use it in the returned `RecordBatch`.

I've also fixed the comments from #3736 that were deemed to be better fixed with this change.

Author: Eric Erhardt <eric.erhardt@microsoft.com>

Closes #3925 from eerhardt/ArrowReaderSync and squashes the following commits:

b48d41be <Eric Erhardt> Use theories in a couple of tests.
23497e20 <Eric Erhardt> ArrowBuffer.Builder should only allocate its current Length, not its current Capacity.
37a9b187 <Eric Erhardt> Fix up the benchmarks.
0b0d9d48 <Eric Erhardt> PR feedback
bd41384f <Eric Erhardt> Respond to initial feedback.
63e33879 <Eric Erhardt> Eliminate allocations in ArrowStreamReader when reading from a Stream