update racket-postgres/ benchmark

| pg9.4 | haskell/ghc7.10.3 | postgresql-simple |       0.61 |       26.6 |

| pg9.4 | rust/rust1.13.0   | postgres          |       0.50 |       26.3 |

| pg9.4 | go/go1.7.3        | pq                |       0.37 |       27.2 |

| pg9.4 | racket/racket6.5  | built-in db       |       2.19 |       26.7 |

| pg9.4 | racket/racket6.5  | built-in db       |       1.99 |       26.7 |

- Remarks:

  - python and go's db binding is as fast as C.

  - rust and haskell is slower. C is 1.65x faster.

  - racket is much slower. C is 6x faster.

  - racket is much slower. C is 5x faster.

** postgres 9.4, cpython 2.7, psycopg2==2.6.1

http://initd.org/psycopg/docs/usage.html

  So write performance is definitely testing disk IO performance. Read

  performance is related to the language and runtime.

- try compile it with raco exe first.

  2.00

  27.84

  --

  2.02

  27.62

  it's about the same.

- try use query-rows instead of query.

  1.78

  26.94

  --

  2.02

  29.08

  --

  1.99

  26.79

  it's still the slowest.

bench

    (display (format "~as\n" (/ (- (current-inexact-milliseconds) start) 1000)))))

(define (select-test)

  (timeit (query pgc "SELECT id, bar FROM foo") TIMES))

  (timeit (query-rows pgc "SELECT id, bar FROM foo") TIMES))

(define (update-test)

  (timeit (query-exec pgc "UPDATE foo SET bar=bar+1 WHERE id=1") TIMES))