Fix connection check assertion during massive-to-live sync transition

After transitioning from massive sync to live mode, the active_connections_before
baseline still contained the old massive sync connections (PostDataCache, Votes,
Follow, etc.) which were closed by Blocks.close_own_db_access(). This caused
the assertion at the end of _process_live_blocks to fail because it compared
the stale baseline (with many connections) against the current state (with only
hivemind_root).

The fix refreshes the baseline connections after setting up the new live mode
database access, ensuring the assertion compares apples to apples.