Hard-cache some IMAP FETCH-related tokens

These get spammed pretty hard with large mailboxes, so it's probably
worth caching them in order to relieve GC pressure.

Part of an ongoing set of fixes for issue #1335

MailKit/Net/Imap/ImapToken.cs

@@ -71,7 +71,24 @@ class ImapToken
 			new ImapToken (ImapTokenType.Nil, "NIL")
 		};
 
-		static readonly Dictionary<string, int> Statistics = new Dictionary<string, int> ();
+		static readonly ImapToken Fetch = new ImapToken (ImapTokenType.Atom, "FETCH");
+		//static readonly ImapToken Annotation = new ImapToken (ImapTokenType.Atom, "ANNOTATION");
+		static readonly ImapToken Body = new ImapToken (ImapTokenType.Atom, "BODY");
+		static readonly ImapToken BodyStructure = new ImapToken (ImapTokenType.Atom, "BODYSTRUCTURE");
+		//static readonly ImapToken EmailId = new ImapToken (ImapTokenType.Atom, "EMAILID");
+		static readonly ImapToken Envelope = new ImapToken (ImapTokenType.Atom, "ENVELOPE");
+		static readonly ImapToken Flags = new ImapToken (ImapTokenType.Atom, "FLAGS");
+		//static readonly ImapToken Header = new ImapToken (ImapTokenType.Atom, "HEADER");
+		//static readonly ImapToken HeaderFields = new ImapToken (ImapTokenType.Atom, "HEADER.FIELDS");
+		static readonly ImapToken InternalDate = new ImapToken (ImapTokenType.Atom, "INTERNALDATE");
+		static readonly ImapToken ModSeq = new ImapToken (ImapTokenType.Atom, "MODSEQ");
+		static readonly ImapToken Rfc822Size = new ImapToken (ImapTokenType.Atom, "RFC822.SIZE");
+		//static readonly ImapToken SaveDate = new ImapToken (ImapTokenType.Atom, "SAVEDATE");
+		//static readonly ImapToken ThreadId = new ImapToken (ImapTokenType.Atom, "THREADID");
+		static readonly ImapToken Uid = new ImapToken (ImapTokenType.Atom, "UID");
+		static readonly ImapToken XGMLabels = new ImapToken (ImapTokenType.Atom, "X-GM-LABELS");
+		static readonly ImapToken XGMMsgId = new ImapToken (ImapTokenType.Atom, "X-GM-MSGID");
+		static readonly ImapToken XGMThrId = new ImapToken (ImapTokenType.Atom, "X-GM-THRID");
 
 		public readonly ImapTokenType Type;
 		public readonly object Value;
@@ -126,13 +143,35 @@ public static ImapToken Create (ImapTokenType type, ByteArrayBuilder builder)
 				NilTokens.Add (nil);
 
 				return nil;
+			} else {
+				if (builder.Equals ("FETCH", false))
+					return Fetch;
+				if (builder.Equals ("BODY", false))
+					return Body;
+				if (builder.Equals ("BODYSTRUCTURE", false))
+					return BodyStructure;
+				if (builder.Equals ("ENVELOPE", false))
+					return Envelope;
+				if (builder.Equals ("FLAGS", false))
+					return Flags;
+				if (builder.Equals ("INTERNALDATE", false))
+					return InternalDate;
+				if (builder.Equals ("MODSEQ", false))
+					return ModSeq;
+				if (builder.Equals ("RFC822.SIZE", false))
+					return Rfc822Size;
+				if (builder.Equals ("UID", false))
+					return Uid;
+				if (builder.Equals ("X-GM-LABELS", false))
+					return XGMLabels;
+				if (builder.Equals ("X-GM-MSGID", false))
+					return XGMMsgId;
+				if (builder.Equals ("X-GM-THRID", false))
+					return XGMThrId;
 			}
 
 			value = builder.ToString ();
 
-			Statistics.TryGetValue (value, out int instances);
-			Statistics[value] = instances + 1;
-
 			return new ImapToken (type, value);
 		}