dialog: Fix data reload race conditions on startup
Since loading the data on child_init(), the load_dialog_info_from_db()
routines and rcv_cluster_event() routines could run in parallel, without
any synchronization on the dialog table, which could lead to duplicate
dialogs in the hash.
dialog: Decrement dialog stats during post-sync cleanup
After a sync completes, it proceeds to clean up all dialogs loaded from
DB which did not match the data received via sync. However, make sure
to also decrement the 'active' / 'early' dialog stats on each delete!