ENH: short-circuit config lookup when exact key given GH5147

[ghost]

Related #5147, #5457

before

In [4]: %timeit -r 10 pd.get_option("display.max_rows")
10000 loops, best of 10: 53.3 us per loop

after

In [1]: %timeit -r 10 pd.get_option("display.max_rows")
100000 loops, best of 10: 5.5 us per loop

jeff's version with no error checking or rerouting deprecated keys

In [1]: %timeit -r 10 pd.core.config._get_option_fast("display.max_rows")
1000000 loops, best of 10: 792 ns per loop

Probably makes no practical difference.