Optimize queries

[larsmans]

Main change is running three SQLite API functions every time we cross the border to Exec a query.

Before:

BenchmarkExec           500000     162257 req/s
BenchmarkQuery          100000      60699 req/s
BenchmarkParams         100000      56971 req/s
BenchmarkStmt           200000      98856 req/s
BenchmarkRows             5000       1883 req/s
BenchmarkStmtRows         5000       1926 req/s
PASS
ok      github.com/mattn/go-sqlite3 18.560s

After:

BenchmarkExec           500000     172340 req/s
BenchmarkQuery          100000      62041 req/s
BenchmarkParams         100000      58299 req/s
BenchmarkStmt           200000     101137 req/s
BenchmarkRows             5000       1922 req/s
BenchmarkStmtRows         5000       1946 req/s
PASS
ok      github.com/mattn/go-sqlite3 18.178s

(These are single benchmarks, but the speedup in BenchmarkExec is consistent across runs.)

[coveralls]

Coverage decreased (-0.15%) to 84.48% when pulling 4dfb2ec on larsmans:optimize into 07f9c9c on mattn:master.

[mattn]

LGTM, Thanks!