Don't allow immediate inv driven block downloads if
a peer already has MAX_BLOCKS_IN_TRANSIT_PER_PEER
active downloads. Prevents bogus inv spam from
blowing up block transfer tracking data structures.
Flushing after every line when printing to console is desirable when
running with systemd but setvbuf() has slightly different semantics
on Windows that causes warnings. Just do an explicit fflush() after
each line print to console instead.