Move the conversion of the WT_MULTI reconciliation information to
WT_REF structures, and the discard of the child page, outside of
the parent lock.  This means the parent lock is hopefully only held
across two memory operations (an allocation and a free), or three
at most, plus an atomic increment/decrement.

Minor rework of the error handling, there's not much that can fail,
and it doesn't matter much if it does.
'parent' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
comment typo
Clean up a comment, the code to delete on-page overflow keys after a
split isn't going away.
comment clarification.
Add some comments to __wt_btree_size_overflow().
The test for removed overflow keys was inverted, fix it, and now overflow
keys work again.
I messed up the eviction test, a tree that can be evicted always
switches.
Merge pull request #941 from wiredtiger/new-split-shorten-lock-region

shorten the parent page lock duration when splitting