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>

npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,webfd/go-zh,npe9/noahevans-go9,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang,d0f/go-zh,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,d0f/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,Triskite/willstone-goclone,Triskite/willstone-goclone,Triskite/willstone-goclone,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,scirelli/scirelli-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,bryanxu/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,bryanxu/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,Triskite/willstone-goclone,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,npe9/noahevans-go9,scirelli/scirelli-go,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang