Fix massive-to-live transition: get baseline after closing connections

The _wait_for_connections_closed call was checking against connections_before
which still contained all massive sync connections (PostDataCache, Votes, etc.).
After Blocks.close_own_db_access() closes these connections, only hivemind_root
remains, causing the assertion to fail since it expected connections to match.

The fix removes the unnecessary _wait_for_connections_closed call and instead
gets a fresh baseline after connections are closed, then uses that baseline
for the assertion at the end of _process_live_blocks.