[MERGE][0.15] Updates to OS Locks to support win32

[Robert Collins]

Robert Collins has voted +1.
Status is now: Approved
Comment:
I think that dirstates use of read locks should be something like this:
when the tree is readlocked, then while the dirstate is accessed, it 
should be read-locked, but if we have cause to do a full-parse of the 
dirstate, we should unlock and close the file: theres no further reason 
to read it, nor have it locked. If we then decide we want to update it, 
we should write lock it, check the checksum at the top is unchanged, and 
proceed to write to it.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45FF0C49.5050804%40arbash-meinel.com%3E