Delay freeing the checkpoint's alloc/discard lists until the resolution step

@agorrod, I'm wondering if we're on the right track with delaying the cleanup of the alloc/discard lists, but the trick is to clear the WT_BTREE.checkpointing flag sooner? This is a branch, it's probably a needless complication to the code if it doesn't help.

Delay freeing the checkpoint's alloc/discard lists until the resolution step, that should help eviction continue sooner.

Reference WT-924.