Split out rephist parts of buckets_decrement

For bufferevents, we had all of connection_buckets_decrement() stubbed
out.  But that's not actually right!  The rephist_* parts were
essential for, inter alia, recording our own bandwidth.  This patch
splits out the rephist parts of connection_buckets_decrement() into their
own function, and makes the bufferevent code call that new function.

Fixes bug 3803, and probably 3824 and 3826 too.  Bugfix on 0.2.3.1-alpha.