With the new addition of the holder background scan, it's possible
for an open holder to write log messages at arbitrary times. The
TestHolder_Open/ErrIndexName test checks the contents of the output
buffer, but those contents could be changing if the background task
happens to run at the right time. Use trivial locking around that
so that this shouldn't happen.
and it moves the file handling (open/close) out of the main Command and
into the interface implementation. The Logger implementations both have
a `Logger()` method which returns their internal logger (`*log.Logger`).
This is because the gossip setup (memberlist) needs a `*log.Logger` for
its configuration.
