Remove unnecessary allocation in ArrowStreamWriter (#73)

## What's Changed

ArrowStreamWriter allocates 8k of memory by creating a new array pool.
This change introduces a shared buffer instead of pool to reduce the
allocations.

The array pool is used to rent small arrays (8 bytes) but the pool
allocates much bigger arrays (8kb in total)
The Array pool has access time overhead for small arrays compared to
direct allocation.

Results from benchmarks:

Old implementation:
| Method | BatchLength | ColumnSetCount | Mean | Error | StdDev |
Allocated |
|----------- |------------ |---------------
|-----------:|----------:|----------:|----------:|
| WriteBatch | 10000 | 10 | 6.118 ms | 0.1215 ms | 0.3345 ms | 248.53 KB
|
| WriteBatch | 10000 | 14 | 9.788 ms | 0.1910 ms | 0.3396 ms | 324.12 KB
|
| WriteBatch | 300000 | 10 | 119.351 ms | 3.1713 ms | 9.3008 ms | 248.53
KB |
| WriteBatch | 300000 | 14 | 136.697 ms | 2.9229 ms | 8.4799 ms | 324.12
KB |

New implementation:
| Method | BatchLength | ColumnSetCount | Mean | Error | StdDev | Median
| Allocated |
|----------- |------------ |---------------
|-----------:|----------:|-----------:|-----------:|----------:|
| WriteBatch | 10000 | 10 | 5.925 ms | 0.2057 ms | 0.6001 ms | 5.843 ms
| 240.64 KB |
| WriteBatch | 10000 | 14 | 8.908 ms | 0.2743 ms | 0.8002 ms | 8.778 ms
| 316.23 KB |
| WriteBatch | 300000 | 10 | 94.835 ms | 1.7872 ms | 3.7699 ms | 93.892
ms | 240.64 KB |
| WriteBatch | 300000 | 14 | 147.995 ms | 3.6873 ms | 10.6975 ms |
144.591 ms | 316.23 KB |

Closes #41.

Author: aik-jahoda

src/Apache.Arrow/Ipc/ArrowFileWriter.cs

@@ -14,6 +14,7 @@
 // limitations under the License.
 
 using System;
+using System.Buffers;
 using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -221,7 +222,7 @@ private void WriteFooter(Schema schema)
 
             // Write footer length
 
-            using (Buffers.RentReturn(4, out Memory<byte> buffer))
+            using (ArrayPool<byte>.Shared.RentReturn(4, out Memory<byte> buffer))
             {
                 int footerLength;
                 checked
@@ -292,7 +293,7 @@ private async Task WriteFooterAsync(Schema schema, CancellationToken cancellatio
 
             cancellationToken.ThrowIfCancellationRequested();
 
-            using (Buffers.RentReturn(4, out Memory<byte> buffer))
+            using (ArrayPool<byte>.Shared.RentReturn(4, out Memory<byte> buffer))
             {
                 int footerLength;
                 checked

src/Apache.Arrow/Ipc/ArrowStreamWriter.cs

@@ -611,8 +611,6 @@ public void Visit(IArrowArray array)
 
         protected Stream BaseStream { get; }
 
-        protected ArrayPool<byte> Buffers { get; }
-
         private protected FlatBufferBuilder Builder { get; }
 
         protected bool HasWrittenSchema { get; set; }
@@ -663,7 +661,6 @@ public ArrowStreamWriter(Stream baseStream, Schema schema, bool leaveOpen, IpcOp
             _leaveOpen = leaveOpen;
             _allocator = allocator ?? MemoryAllocator.Default.Value;
 
-            Buffers = ArrayPool<byte>.Create();
             Builder = new FlatBufferBuilder(1024);
             HasWrittenSchema = false;
 
@@ -1277,7 +1274,7 @@ private protected async ValueTask WriteFlatBufferAsync(CancellationToken cancell
 
         private void WriteIpcMessageLength(int length)
         {
-            using (Buffers.RentReturn(_options.SizeOfIpcLength, out Memory<byte> buffer))
+            using (ArrayPool<byte>.Shared.RentReturn(_options.SizeOfIpcLength, out Memory<byte> buffer))
             {
                 Memory<byte> currentBufferPosition = buffer;
                 if (!_options.WriteLegacyIpcFormat)
@@ -1294,7 +1291,7 @@ private void WriteIpcMessageLength(int length)
 
         private async ValueTask WriteIpcMessageLengthAsync(int length, CancellationToken cancellationToken)
         {
-            using (Buffers.RentReturn(_options.SizeOfIpcLength, out Memory<byte> buffer))
+            using (ArrayPool<byte>.Shared.RentReturn(_options.SizeOfIpcLength, out Memory<byte> buffer))
             {
                 Memory<byte> currentBufferPosition = buffer;
                 if (!_options.WriteLegacyIpcFormat)