fix(pool): roll back failed submit tracking

Co-Authored-By: Virgil <virgil@lethean.io>

pool/impl.go

@@ -138,7 +138,11 @@ func (c *StratumClient) Submit(jobID, nonce, result, algo string) int64 {
 			"algo":   algo,
 		},
 	}
-	_ = c.writeJSON(req)
+	if err := c.writeJSON(req); err != nil {
+		c.mu.Lock()
+		delete(c.pending, seq)
+		c.mu.Unlock()
+	}
 	return seq
 }