deprecate indicator_max_period_candles, automatically compute startup candles for FreqAI backtesting.

config_examples/config_freqai.example.json

@@ -9,6 +9,7 @@
     "dry_run": true,
     "timeframe": "3m",
     "dry_run_wallet": 1000,
+    "startup_candle_count": 20,
     "cancel_open_orders_on_exit": true,
     "unfilledtimeout": {
         "entry": 10,
@@ -53,7 +54,6 @@
     ],
     "freqai": {
         "enabled": true,
-        "startup_candles": 10000,
         "purge_old_models": true,
         "train_period_days": 15,
         "backtest_period_days": 7,