[ccache] Buffer size for IO operations is too small

[Michael Kolomeytsev]

I've discovered that there is too small buffer size for IO in ccache: 16k
or 10k
(in hash_fd, copy_fd, copy_file).

I did simple fix and run several tests (on mac osx) trying to recompile
chromium.
(Of course there was 100% cache hit).
Results (pay your attention to sys time):

16k

real 8m47.043s

user 47m43.881s

sys 13m29.614s


32k

real 8m35.696s

user 47m27.329s

sys 12m23.814s


64k

real 8m33.496s

user 47m31.393s

sys 12m0.532s


128k

real 8m31.473s

user 47m45.633s

sys 11m53.050s


256k

real 8m30.719s

user 47m54.551s

sys 11m51.810s


512k

real 8m39.989s

user 48m7.429s

sys 12m11.997s


1m

real 8m44.547s

user 48m20.203s

sys 12m36.895s


2m

real 8m48.912s

user 48m34.397s

sys 12m59.308s

I'm going to make an PR at github.