Introduce spider_sync_stats_enabled plugin variable

Introduce the variable spider_sync_stats that replaces spider_crd_bg_mode, spider_crd_sync, spider_bg_sts_mode, spider_sts_sync.

Spider synchronizes table and index statistics in the background if and only if spider_stats_sync = 1. No other tweaks, which could be done with the old variables, are necessary. Do not forget to delete the dead code.