Start tuning the WT_SESSION_IMPL cache of WT_EXT and WT_SIZE structures.

@michaelcahill:

Increase the pre-alloc of WT_EXT/WT_SIZE structures from 5/2 to 5/5 in the write/free paths, and from 100/10 to 250/250 in the checkpoint path.

Turn off cache limits: checkpoints can take tens of thousands of WT_EXT structures and hundreds of WT_SIZE structures, we want to cache everything until the checkpoint completes.  Add a cleanup call after the checkpoint so we don't tie down megabytes of memory between checkpoints.

Split the previous pre-allocate function into two parts, it's simpler as a function to pre-allocate entries and a separate function to take an entry for use.

Remove the tracking of "memory allocations while locked"; there are extent lists we read while not holding any locks, and they can be really, really large requiring the allocation of tens of thousands of WT_EXT structures, they make the statistic meaningless.