perf: Replace channel check with atomic bool in tailer.send()

[benclive]

What this PR does / why we need it:

• @cyriltovena discovered that tailer.isClosed was eating a significant amount of CPU in ops (>20 CPUs worth)
  ]
• I replaced the select on the channel to try and reduce the amount of CPU time spent on this particular code path.
• I used an atomic bool for the send() but retained the closeChan to signal the other parts of the tailer to shut down ASAP, otherwise we'd need to check the bool periodically using (for example) time.After.

Benchmark before thte change, using select-default:

Benchmark_tailer_isClosed
Benchmark_tailer_isClosed-14    	536670529	         2.220 ns/op
PASS

Benchmark after the change, using atomic bool:

Benchmark_tailer_isClosed
Benchmark_tailer_isClosed-14    	1000000000	         0.2886 ns/op
PASS

So this small change is nearly 10x faster and therefore would save ~18-20 cores worth of CPU time in our ops cluster.

[MasslessParticle]

Would it make sense to remove closeChan altogether? I see it's used in a few other select statements that we can't get rid of, but it would keep us from signaling closed in two different ways in the same file.

Nevermind. On closer look, those other usages rely on the close chan to interrupt selects blocking

[cyriltovena]

LGTM !

For next time, in your PR you can use benchstat it gives a much better output.