Attach cause to ComException on Client.start

This is to enable more investigation of a rare flaky failure of IndexOperationsIT.
Revert "Debug logging of cluster members state"

This reverts commit 89f93d3c986efe18574bb353453195893f8f0b9e.
Cope better when asking import-tool to import empty files

* ImportTool now no longer prints a big stack trace (by default) when asked to import empty files.
* Add a new --stacktrace command line option that can enable printing of stack
  traces for those who want to debug things.
* The Thread created by the ThreadReadaheadReadable has been made daemon so it
  no longer prevents a shut down of the importer process.
* Panics in the import pipeline now preserve exception messages when wrapping
  causes.
Merge pull request #3567 from chrisvest/2.2-import-empty-files

Cope better when asking import-tool to import empty files
Add missing entry for import in manpage list.
Merge pull request #3559 from chrisvest/2.2-indexops-it

Attach cause to ComException on Client.start
Merge pull request #3566 from lutovich/2.2-remove-debug-logging

Revert "Debug logging of cluster members state"
Merge pull request #3572 from nawroth/import-manpage

Add missing entry for import in manpage list.