Do not perform R/O ops in R/W ctx (in a transaction). Updates #20.

benchmark             old ns/op    new ns/op    delta
BenchmarkDelete16        323566       316010   -2.34%
BenchmarkExtract16       321315       311687   -3.00%
BenchmarkFirst16          15918        14149  -11.11%
BenchmarkGet16           231509       188653  -18.51%
BenchmarkInc              19391        18345   -5.39%
BenchmarkLast16           12450        12420   -0.24%
BenchmarkPut16           301181       286454   -4.89%
BenchmarkSet16           237416       251551   +5.95%
BenchmarkSeek            222719       191099  -14.20%
BenchmarkNext1e3        1507441       531415  -64.75%

benchmark            old allocs   new allocs    delta
BenchmarkDelete16            62           62    0.00%
BenchmarkExtract16           61           61    0.00%
BenchmarkFirst16              8            4  -50.00%
BenchmarkGet16               63           19  -69.84%
BenchmarkInc                 10           10    0.00%
BenchmarkLast16               8            4  -50.00%
BenchmarkPut16               50           50    0.00%
BenchmarkSet16               43           45    4.65%
BenchmarkSeek                64           20  -68.75%
BenchmarkNext1e3           7027         3000  -57.31%

benchmark             old bytes    new bytes    delta
BenchmarkDelete16         33226        32941   -0.86%
BenchmarkExtract16        23711        23590   -0.51%
BenchmarkFirst16          25070        24958   -0.45%
BenchmarkGet16           169886       144599  -14.88%
BenchmarkInc                944          944    0.00%
BenchmarkLast16           20974        20862   -0.53%
BenchmarkPut16            19082        18792   -1.52%
BenchmarkSet16            14872        15690    5.50%
BenchmarkSeek            156007       130807  -16.15%
BenchmarkNext1e3         141331        24080  -82.96%