net: use stronger AddLocal() for our I2P address

There are two issues:

1. Our I2P address not added to local addresses.

* `externalip=` is used with an IPv4 address (this sets automatically
  `discover=0`)
* No `discover=1` is used
* `i2psam=` is used
* No `externalip=` is used for our I2P address
* `listenonion=1 torcontrol=` are used

In this case `AddLocal(LOCAL_MANUAL)` is used for our `.onion` address
and `AddLocal(LOCAL_BIND)` for our `.b32.i2p` address, the latter being
ignored due to `discover=0`.

2. Our I2P address removed from local addresses even if specified
with `externalip=` on I2P proxy restart.

* `externalip=` is used with our I2P address (this sets automatically
  `discover=0`)
* No `discover=1` is used
* `i2psam=` is used

In this case, initially `externalip=` causes our I2P address to be added
with `AddLocal(LOCAL_MANUAL)` which overrides `discover=0` and works as
expected. However, if later the I2P proxy is shut down we do
`RemoveLocal()` in order to stop advertising our I2P address (since we
have lost I2P connectivity). When the I2P proxy is started and we
reconnect to it, restoring the I2P connectivity, we do
`AddLocal(LOCAL_BIND)` which does nothing due to `discover=0`.

To resolve those two issues, use `AddLocal(LOCAL_MANUAL)` for I2P which
is also what we do with Tor.

src/net.cpp

@@ -2244,7 +2244,7 @@ void CConnman::ThreadI2PAcceptIncoming()
         }
 
         if (!advertising_listen_addr) {
-            AddLocal(conn.me, LOCAL_BIND);
+            AddLocal(conn.me, LOCAL_MANUAL);
             advertising_listen_addr = true;
         }