Deprecate :{start,end}_time, add :timing instead

[kyrylo]

There are a few reasons for this change:

1. We use realtime to measure performance for the :total stat. This is no
   good, especially since we use monotonic time for other groups. It is
   inconsitent and confusing

2. New functionality requires a better API. Specifically, we want to be able to
   send timing of a certain job. To achieve that we wrap the job in a benchmark
   measure call. The problem is that our API doesn't make it easy to send
   already calculated timing. start_time makes the user code very clunky and
   confusing

Before:

start_time = Time.now
calculate_operation

Airbrake.notify_request(
  ...,
  start_time: start_time,
  end_time: Time.now,
)

After:

timing = Airbrake::Benchmark.measure do
  calculate_operation
end

Airbrake.notify_request(
  ...,
  timing: timing,
)