Experimental change to avoid unpacking cells in order to access
row-store leaf page keys.

If a page has no Huffman or prefix compression, and no overflow keys,
encode the key's page offset and size in the WT_ROW pointer and set a
flag on the page.  From that point on, when the flag is set, we can
go directly to the key instead of unpacking the key's cell each time.
Merge branch 'develop' into avoid-cell-unpack
Merge branch 'develop' into avoid-cell-unpack
Merge branch 'develop' into avoid-cell-unpack
Instead of byte-copying the WT_IKEY structure and data, call the
standard underlying supporting function.  Clang is complaining about the
internal page binary search function going off the end of the allocated
memory: I don't see a bug here, but there's no reason to do the work in
a non-standard way, and maybe I'm just missing something.
Merge branch 'develop' into avoid-cell-unpack

Conflicts:
	src/btree/rec_split.c
Merge branch 'develop' into avoid-cell-unpack

Conflicts:
	src/include/btmem.h
Merge branch 'develop' into avoid-cell-unpack
Merge pull request #1038 from wiredtiger/avoid-cell-unpack

Avoid row-store leaf page key cell unpack