Repair old performance bug in tuplesort.c/logtape.c. In the case where

we are doing the final merge pass on-the-fly, and not writing the data
back onto a 'tape', the number of free blocks in the tape set will become
large, leading to a lot of time wasted in ltsReleaseBlock().  There is
really no need to track the free blocks anymore in this state, so add a
simple shutoff switch.  Per report from Stefan Kaltenbrunner.