database/sql: use slices rather than container/list

Significantly reduces the number of allocations, while also
simplifying the code and increasing performance by a 1-2%.

benchmark                          old ns/op     new ns/op     delta
BenchmarkConcurrentDBExec          13290567      13026236      -1.99%
BenchmarkConcurrentStmtQuery       13249399      13008879      -1.82%
BenchmarkConcurrentStmtExec        8806237       8680182       -1.43%
BenchmarkConcurrentTxQuery         13628379      12756293      -6.40%
BenchmarkConcurrentTxExec          4794800       4722440       -1.51%
BenchmarkConcurrentTxStmtQuery     5040804       5200721       +3.17%
BenchmarkConcurrentTxStmtExec      1366574       1336626       -2.19%
BenchmarkConcurrentRandom          11119120      10926113      -1.74%

benchmark                          old allocs     new allocs     delta
BenchmarkConcurrentDBExec          14191          13684          -3.57%
BenchmarkConcurrentStmtQuery       16020          15514          -3.16%
BenchmarkConcurrentStmtExec        4179           3672           -12.13%
BenchmarkConcurrentTxQuery         16025          15518          -3.16%
BenchmarkConcurrentTxExec          12717          12709          -0.06%
BenchmarkConcurrentTxStmtQuery     15532          15525          -0.05%
BenchmarkConcurrentTxStmtExec      2175           2168           -0.32%
BenchmarkConcurrentRandom          12320          11997          -2.62%

benchmark                          old bytes     new bytes     delta
BenchmarkConcurrentDBExec          2164827       2139760       -1.16%
BenchmarkConcurrentStmtQuery       2418070       2394030       -0.99%
BenchmarkConcurrentStmtExec        1728782       1704371       -1.41%
BenchmarkConcurrentTxQuery         2477144       2452620       -0.99%
BenchmarkConcurrentTxExec          588920        588343        -0.10%
BenchmarkConcurrentTxStmtQuery     790866        796578        +0.72%
BenchmarkConcurrentTxStmtExec      98502         98143         -0.36%
BenchmarkConcurrentRandom          1725906       1710220       -0.91%

LGTM=ruiu, dave, bradfitz
R=golang-codereviews, ruiu, gobot, bradfitz, dave, minux
CC=bradfitz, golang-codereviews
https://golang.org/cl/107020044