Try to merge split-merge pages to minimize deepening the in-memory tree.
Don't walk up past the root looking for split-merge pages to force evict.
Merge branch 'develop' into merge-tree.
btree/bt_merge.c:126: warning: implicit conversion shortens 64-bit value into a 32-bit value
Merge branch 'develop' into merge-tree
Cleanup and additional comments based on Sue's review.
refs #450
Backout one of the review changes: don't use WT_LINK_PAGE everywhere.
Only consider split-merge pages during forced eviction if the stack is deep enough.
When merging, optimize for the case where the first or last child is the only "live" one, and avoid growing a level for the live node.
When searching internal nodes, fast-path appends.
whitespace, remove duplicated line.
Call out the remaining WT_REF states explicitly, lint complains.
__evict_init_candidate() can't fail, change it to return void.
Minor fix to a comment, WT_REC_SINGLE was replaced by the exclusive arg.
Cleanup based on Keith's review feedback.
Fast-path searching of row store internal pages with a single child.
Rename bt_merge.c to rec_merge.c.

--HG--
rename : src/btree/bt_merge.c => src/btree/rec_merge.c
Add a comment about LRU priorities for split-merge pages.
Simplify __merge_walk: we only call it twice, it doesn't need to be completely general.
Add more stats for merge: total levels collapsed, unsuccessful attempts.