Modified ByteArrayBuilder.Clear() to downsize its buffer if it has gr…

…own too large

One potential problem with reusing ByteArrayBuilders is that, because they can
grow for some abnormally long tokens/lines/commands/etc, those oversized buffers
will remain referenced by the ImapStream/ImapEngine until they are disposed which
could be the life of the program.

If we oportunistically scale back the size of the buffers when they are Clear()'d,
then we can periodically reduce memory usage and allow those larger buffers to be
used elsewhere.

MailKit/ByteArrayBuilder.cs

@@ -27,17 +27,20 @@
 using System;
 using System.Text;
 using System.Buffers;
+using System.Runtime.CompilerServices;
 
 namespace MailKit
 {
 	class ByteArrayBuilder : IDisposable
 	{
+		readonly int initialCapacity;
 		byte[] buffer;
 		int length;
 
-		public ByteArrayBuilder (int initialCapacity)
+		public ByteArrayBuilder (int capacity)
 		{
-			buffer = ArrayPool<byte>.Shared.Rent (initialCapacity);
+			buffer = ArrayPool<byte>.Shared.Rent (capacity);
+			initialCapacity = capacity;
 			length = 0;
 		}
 
@@ -54,6 +57,7 @@ public byte[] GetBuffer ()
 			return buffer;
 		}
 
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
 		void EnsureCapacity (int capacity)
 		{
 			if (capacity > buffer.Length) {
@@ -79,6 +83,11 @@ public void Append (byte[] text, int startIndex, int count)
 
 		public void Clear ()
 		{
+			if (buffer.Length > initialCapacity * 4) {
+				ArrayPool<byte>.Shared.Return (buffer);
+				buffer = ArrayPool<byte>.Shared.Rent (initialCapacity);
+			}
+
 			length = 0;
 		}