Add a RMW insert mode to wtbench. Fix build warnings:

../src/btree/rec_merge.c: In function '__merge_transfer_footprint':
../src/btree/rec_merge.c:154: warning: implicit conversion shortens
64-bit value into a 32-bit value
../src/btree/rec_merge.c:155: warning: implicit conversion shortens
64-bit value into a 32-bit value