i2p: lower the number of tunnels for transient sessions

This will lower the load on the I2P network. Since we use one transient
session for connecting to just one peer, a higher number of tunnels is
unnecessary.

This was suggested in:
bitcoin/bitcoin#26754 (comment)
bitcoin/bitcoin#26754 (comment)

The options are documented in:
https://geti2p.net/en/docs/protocol/i2cp#options

A tunnel is unidirectional, so even if we make a single outbound
connection we still need an inbound tunnel to receive the messages sent
to us over that connection.

Alleviates: bitcoin/bitcoin#26754

Author: vasild

src/i2p.cpp

@@ -380,7 +380,9 @@ void Session::CreateIfNotCreatedAlready()
         // in the reply in DESTINATION=.
         const Reply& reply = SendRequestAndGetReply(
             *sock,
-            strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=TRANSIENT SIGNATURE_TYPE=7", session_id));
+            strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=TRANSIENT SIGNATURE_TYPE=7 "
+                      "inbound.quantity=1 outbound.quantity=1",
+                      session_id));
 
         m_private_key = DecodeI2PBase64(reply.Get("DESTINATION"));
     } else {