time: fix tick accuracy when using multiple Tickers
time: fix tick accuracy when using multiple Tickers

* correctly ignore obsolete ticks
* fix update of next alarm time
* tighten alarm channel buffers
Fixes issue 1379.

R=r, adg, r2
CC=golang-dev, soul9
http://codereview.appspot.com/3789045

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go