Keep limited base history.

As per RFC 6817:
> To implement an approximate minimum over the past few minutes, a
> LEDBAT sender stores BASE_HISTORY separate minima -- one each for the
> last BASE_HISTORY-1 minutes, and one for the running current minute.
> At the end of the current minute, the window moves -- the earliest
> minimum is dropped and the latest minimum is added.

This also slightly improves performance.