Add a wtperf config that disables prefix compression, to see how the results compare.

--HG--
rename : bench/wtperf/runners/medium-multi-lsm.wtperf => bench/wtperf/runners/medium-multi-lsm-noprefix.wtperf
Disable LSM salvage in test/format until ref #972 is fixed.
Have the LSM checkpoint thread continue on EBUSY, until ref #973 is fixed.
../src/lsm/lsm_worker.c:404:6: error: 'j' may be used uninitialized in this function [-Werror=maybe-uninitialized]

refs #973