i2p: also sleep after errors in Accept()

Background:

`Listen()` does:
* if the session is not created yet
  * create the control socket and on it:
  * `HELLO`
  * `SESSION CREATE ID=sessid`
  * leave the control socked opened
* create a new socket and on it:
* `HELLO`
* `STREAM ACCEPT ID=sessid`
* read reply (`STREAM STATUS`)

Then a wait starts, for a peer to connect. When connected,

`Accept()` does:
* on the socket from `STREAM ACCEPT` from `Listen()`: read the
  Base64 identification of the connecting peer

Problem:

The I2P router may be in such a state that this happens in a quick
succession (many times per second, see bitcoin/bitcoin#22759 (comment)):
`Listen()`-succeeds, `Accept()`-fails.

`Accept()` fails because the I2P router sends something that is
not Base64 on the socket:
STREAM STATUS RESULT=I2P_ERROR MESSAGE="Session was closed"

We only sleep after failed `Listen()` because the assumption was that
if `Accept()` fails then the next `Listen()` will also fail.

Solution:

Avoid filling the log with "Error accepting:" messages and sleep also
after a failed `Accept()`.

Extra changes:

* Reset the error waiting time after one successful connection.
  Otherwise the timer will remain high due to problems that have
  vanished long time ago.

* Increment the wait time less aggressively.

Author: vasild

src/net.cpp

@@ -2974,19 +2974,21 @@ void CConnman::ThreadI2PAcceptIncoming()
     bool advertising_listen_addr = false;
     i2p::Connection conn;
 
+    auto SleepOnFailure = [&]() {
+        interruptNet.sleep_for(err_wait);
+        if (err_wait < err_wait_cap) {
+            err_wait += 1s;
+        }
+    };
+
     while (!interruptNet) {
 
         if (!m_i2p_sam_session->Listen(conn)) {
             if (advertising_listen_addr && conn.me.IsValid()) {
                 RemoveLocal(conn.me);
                 advertising_listen_addr = false;
             }
-
-            interruptNet.sleep_for(err_wait);
-            if (err_wait < err_wait_cap) {
-                err_wait *= 2;
-            }
-
+            SleepOnFailure();
             continue;
         }
 
@@ -2996,11 +2998,14 @@ void CConnman::ThreadI2PAcceptIncoming()
         }
 
         if (!m_i2p_sam_session->Accept(conn)) {
+            SleepOnFailure();
             continue;
         }
 
         CreateNodeFromAcceptedSocket(std::move(conn.sock), NetPermissionFlags::None,
                                      CAddress{conn.me, NODE_NONE}, CAddress{conn.peer, NODE_NONE});
+
+        err_wait = err_wait_begin;
     }
 }