Don't return early from VSL_Dispatch when cursor reports more data available

VSL_Dispatch would return with (1) when the cursor reports more data
available, causing the processing of ready transaction not to be
run. Would cause 'varnishlog -d' not to report any data in some
situations.

Add some comments about the different return points in VSL_Dispatch.
Count transaction limit towards the number of outstanding transactions
instead of the number of incomplete transactions.

Session grouping mode could grab an unintended amount of memory
because the transaction limit was counted towards the incomplete
rather that not yet reported transactions. A long lasting session
would only count as 1 towards the limit even if it had thousands of
complete children that should put it above the limit.