When adding log blocks, trigger flush also based on available disk blocks,
that is, do not add blocks to the logbuffer which we can not also flush.

Also flush with reference:

I think the capability was originally limited in order to do
full flushes with reference only from the logwatcher thread, in
order to not hold the logmtx for too long.

But now that we have the extra flush finish thread, I do not think
this is necessary any more, and we need to handle tight storage
better.