i2p: use consistent number of tunnels with i2pd and Java I2P

The default number of tunnels in the Java implementation is 2 and in the
C++ i2pd it is 5. Pick a mid-number (3) and explicitly set it in order
to get a consistent behavior with both routers. Do this for persistent
sessions which are created once at startup and can be used to open up
to ~10 outbound connections and can accept up to ~125 incoming
connections. Transient sessions already set number of tunnels to 1.

Suggested in:
bitcoin/bitcoin#26754 (comment)
https://geti2p.net/en/docs/api/samv3

Alleviates: bitcoin/bitcoin#26754

Author: vasild

src/i2p.cpp

@@ -398,7 +398,8 @@ void Session::CreateIfNotCreatedAlready()
         const std::string& private_key_b64 = SwapBase64(EncodeBase64(m_private_key));
 
         SendRequestAndGetReply(*sock,
-                               strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=%s",
+                               strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=%s "
+                                         "inbound.quantity=3 outbound.quantity=3",
                                          session_id,
                                          private_key_b64));
     }