Made ta_on_candle and optional

No change to existing bot behaviour.
IF a user sets `ta_on_candle": true,` in their config.json
then only when new candle-for-pair is recieved are TA functions called.

freqtrade/constants.py

@@ -53,6 +53,7 @@ CONF_SCHEMA = {
         },
         'fiat_display_currency': {'type': 'string', 'enum': SUPPORTED_FIAT},
         'dry_run': {'type': 'boolean'},
+        'ta_on_candle': {'type': 'boolean'},
         'minimal_roi': {
             'type': 'object',
             'patternProperties': {