Fix panic in tailer due to race condition between send() and close()

[pracucci]

What this PR does / why we need it:

Fix a panic in the ingester tailer when send() and close() are executed at the same time.

Which issue(s) this PR fixes:
Fixes #942

Special notes for your reviewer:

• Refactoring: renamed done into closeChan to make it more clear it's a channel used to signal the close()
• Refactoring: changed closeMtx in closeOnce to make it more clear it's just used to run the close() once
• The test fails with the previous logic

Checklist

• Documentation added
• Tests updated

[cyriltovena]

LGTM