time: make timers heap 4-ary

This slightly improves performance when a lot of timers are present

$ misc/benchcmp ../old_timers_m.txt ../new_timers_m.txt
benchmark                           old ns/op    new ns/op    delta
BenchmarkAfterFunc                       6884         6605   -4.05%
BenchmarkAfterFunc-2                     4473         4144   -7.36%
BenchmarkAfterFunc-3                     8601         6185  -28.09%
BenchmarkAfterFunc-4                     9378         8773   -6.45%
BenchmarkAfter                           7237         7278   +0.57%
BenchmarkAfter-2                         4638         3923  -15.42%
BenchmarkAfter-3                         8751         6239  -28.71%
BenchmarkAfter-4                         9223         8737   -5.27%
BenchmarkStop                             603          496  -17.74%
BenchmarkStop-2                           795          577  -27.42%
BenchmarkStop-3                           982          680  -30.75%
BenchmarkStop-4                          1164          739  -36.51%
BenchmarkSimultaneousAfterFunc            657          593   -9.74%
BenchmarkSimultaneousAfterFunc-2          816          757   -7.23%
BenchmarkSimultaneousAfterFunc-3          844          830   -1.66%
BenchmarkSimultaneousAfterFunc-4          785          771   -1.78%
BenchmarkStartStop                        238          239   +0.42%
BenchmarkStartStop-2                      249          234   -6.02%
BenchmarkStartStop-3                      271          268   -1.11%
BenchmarkStartStop-4                      293          295   +0.68%

R=golang-dev, dvyukov, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/13094043

Committer: Dmitriy Vyukov <dvyukov@google.com>