Use leveldb database lock for concurrent upgrade protection (fixes #703)

Doesn't work if config directories are different though
Use JoinHostPort for URL that browser opens (fixes #732)
Add "cluster introducer" functionality to nodes (ref #120)
Add CPU usage tracker for Windows (fixes #729)
Merge remote-tracking branch 'origin/pr/731'

* origin/pr/731:
  Use leveldb database lock for concurrent upgrade protection (fixes #703)
Translation update
Revert "Merge branch 'pr/711'" (...)

Temporary revert to the old debounce behavior. It's a bit bad and drives
up CPU usage, but mostly shows correct info in the GUI. This will be
improved shortly.

This reverts commit 51443308074456ef8f6ca62b29de0bc7ed3aad85, reversing
changes made to c34f3defe1aa1fe28aca1d5e69b712d4d857da49.

Conflicts:
	auto/gui.files.go
Name is not always the hostname
Move top level packages to internal.