Remove call to time.Now() on worker thread when handling record reqs

[ian-mi]

Time is already recorded on the client side and stored in the currently unused recordReq.t
field. Avoiding these repeated calls to time.Now while the worker is blocked can significantly
reduce worker contention.

I observed the following results on the existing benchmarks with this change:

name                 old time/op    new time/op    delta
RecordReqCommand-16    2.91µs ± 4%    2.47µs ± 4%  -15.08%  (p=0.000 n=20+20)
RecordViaStats-16      3.52µs ± 5%    3.06µs ± 3%  -13.00%  (p=0.000 n=20+20)

name                 old alloc/op   new alloc/op   delta
RecordReqCommand-16      426B ± 0%      426B ± 0%     ~     (all equal)
RecordViaStats-16        634B ± 0%      634B ± 0%     ~     (all equal)

name                 old allocs/op  new allocs/op  delta
RecordReqCommand-16      25.0 ± 0%      25.0 ± 0%     ~     (all equal)
RecordViaStats-16        29.0 ± 0%      29.0 ± 0%     ~     (all equal)

In addition when profiling other benchmarks where worker contention proved to be a bottleneck I've observed cases where around half of the worker's time was spent calling time.Now(), e.g.:

(pprof) top view.*start -cum
Active filters:
   focus=view.*start
Showing nodes accounting for 3.75s, 10.45% of 35.90s total
Dropped 42 nodes (cum <= 0.18s)
Showing top 10 nodes out of 58
      flat  flat%   sum%        cum   cum%
         0     0%     0%      5.72s 15.93%  go.opencensus.io/stats/view.(*worker).start
     0.27s  0.75%  0.75%      5.03s 14.01%  go.opencensus.io/stats/view.(*recordReq).handleCommand
     0.18s   0.5%  1.25%      2.88s  8.02%  time.Now
     0.08s  0.22%  1.48%      2.70s  7.52%  time.now
     1.34s  3.73%  5.21%      1.41s  3.93%  runtime.walltime (inline)
     1.15s  3.20%  8.41%      1.21s  3.37%  runtime.nanotime (inline)
     0.03s 0.084%  8.50%      0.96s  2.67%  go.opencensus.io/stats/view.(*viewInternal).addSample
     0.58s  1.62% 10.11%      0.81s  2.26%  runtime.mapiternext
     0.03s 0.084% 10.19%      0.73s  2.03%  go.opencensus.io/stats/view.encodeWithKeys
     0.09s  0.25% 10.45%      0.69s  1.92%  runtime.selectgo