[FIXED JENKINS-34213] Ensure that the unexporter cleans up whatever i…

…t can each sweep (#81)

* [FIXED JENKINS-34213] Ensure that the unexporter cleans up whatever it can each sweep

* [JENKINS-34213] Collect and report meaningful stats, batch the reference collection for better stability

- We want to report on these things only if they are an issue. Logging of the actual stats should be below the radar of the users using a default logger level of `INFO` provided that the Unexporter is not doing much
- When the Unexporter is busy (i.e. the m1 rate is > 100/sec) then we should start reporting at `INFO`
- In the event that there is sustained high levels of work, we should alert the user and recommend turning off the stack traces to reduce GC pressure

- My stress testing revealed that under very heavy load it is better to batch the removal and then batch the clean-up even if this batching means that the sweeps are not as frequent.

* [JENKINS-34213] Oops typo in the stats measurement

* Ignore when the channel is already closed due to a race between testing for close and close

Spotted in https://jenkins.ci.cloudbees.com/job/libraries/job/remoting/177/console

* [JENKINS-34213] Copy and paste error

The three most common programming errors are:

- Off by one errors
- Copy and paste errors

* [JENKINS-34213] Just when you were least expecting it, findbugs finds an actual real bug