[merge] add lock debugging, remove unnecessary lock peeks

[John Arbash Meinel]

John Arbash Meinel has voted -0.
Status is now: Semi-approved
Comment:
I think I like the lock tracing

but I'm worried about getting rid of the extra peek. Launchpad 
specifically was violating our "rename dir => existing_dir" constraint. 
Which meant that we would have been "acquiring" the lock incorrectly, 
and possibly corrupting data.

LP has been fixed, but I don't know about regressions/other FS which 
might do the same. (Like WebDAV, or just a specific implementation, etc)

Because we have a specific example of where it has been necessary, I 
think I would rather keep it.

However, if Robert/Aaron/Martin feel like it is worth the performance 
boost, I won't block (hence -0 not -1).


For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Ce01316480706270302l25fee23ahcf2d39312cff3822%40mail.gmail.com%3E