histogram: calculate bin position of a value in the histrogram

Generate a lower and upper bound.

Signed-off-by: Sage Weil <sage@inktank.com>
histogram: rename set -> set_bin

Signed-off-by: Sage Weil <sage@inktank.com>
histogram: move to common, add unit tests

Signed-off-by: Sage Weil <sage@inktank.com>
wip