Reduce turbotunnel queueSize from 2048 to 512.

This is to reduce heap usage.

tpo/anti-censorship/pluggable-transports/snowflake#40179

Past discussion of queueSize:
https://lists.torproject.org/pipermail/anti-censorship-team/2021-July/000188.html
tpo/anti-censorship/pluggable-transports/snowflake!48 (comment 2744619)

common/turbotunnel/consts.go

@@ -11,7 +11,7 @@ import "errors"
 var Token = [8]byte{0x12, 0x93, 0x60, 0x5d, 0x27, 0x81, 0x75, 0xf5}
 
 // The size of receive and send queues.
-const queueSize = 2048
+const queueSize = 512
 
 var errClosedPacketConn = errors.New("operation on closed connection")
 var errNotImplemented = errors.New("not implemented")