Fix API usage

Your read tests didn't use read-only TXNs, so they showed no scaling for parallel reads. With these fixed, on my AMD quadcore laptop I get:

Wrote 10000 items in 0.012149 sec, 823113 ops/s
Write Parallel [2] 10000 items in 0.011838 sec, 844737 ops/s
Write Parallel [4] 10000 items in 0.021464 sec, 465896 ops/s
Write Parallel [8] 10000 items in 0.017587 sec, 568601 ops/s
Write Parallel [16] 10000 items in 0.015424 sec, 648340 ops/s
Read 10000 items in 0.004716 sec, 2120441 ops/s
Read Parallel [2] 20000 items in 0.012886 sec, 1552072 ops/s
Read Parallel [4] 40000 items in 0.014793 sec, 2703981 ops/s
Read Parallel [8] 80000 items in 0.016813 sec, 4758222 ops/s
Read Parallel [16] 160000 items in 0.030883 sec, 5180843 ops/s
Wrote 10000 items in 0.016397 sec, 609867 ops/s
Read 10000 items in 0.004939 sec, 2024701 ops/s
Wrote 10000 items in 0.347584 sec, 28770 ops/s
Read 10000 items in 0.003645 sec, 2743484 ops/s
Wrote 10000 items in 0.356129 sec, 28079 ops/s
Read 10000 items in 0.003839 sec, 2604845 ops/s