Rework backtesting --no-cahche to --cache=[none, day, week, month].

Fix an issue where config modification during runtime would prevent use of cached results.

freqtrade/commands/cli_options.py

@@ -205,10 +205,12 @@ AVAILABLE_CLI_OPTIONS = {
         nargs='+',
         choices=constants.BACKTEST_BREAKDOWNS
     ),
-    "no_backtest_cache": Arg(
-        '--no-cache',
-        help='Do not reuse cached backtest results.',
-        action='store_true'
+    "backtest_cache": Arg(
+        '--cache',
+        help='Load a cached backtest result no older than specified age.',
+        metavar='AGE',
+        default=constants.BACKTEST_CACHE_DEFAULT,
+        choices=constants.BACKTEST_CACHE_AGE,
     ),
     # Edge
     "stoploss_range": Arg(