Delay setting leak memory flag until after the final checkpoint in shutdown

Mongodb can see a large increase in memory consumption during shutdown. WT-6229 identified and addressed a large part of this problem, but also observed that even with the fix, mongod could increase in size from ~400MB to ~1GB at shutdown (when is sets the WT_CONN_LEAK_MEMORY flag).  

We should continue investigating to understand if there is another leak, or (as in WT-6229) there are some pages that we are repeatedly evicting and rereading.

Eric Milkie provides a simple way to produce the growth-at-shutdown problem in BF-17346.