other forms of LockDir: changing instantiation

[Scott Parish]

I'm working on a new transport for use with bzr (amazon's s3) which
doesn't have an atomic rename--i'm going to have to make a new
implementation of LockDir.  Unfortunately there's no good way to 
plug-in new versions of LockDir.

Right now LockDir is instantiated like:

   lockdir.LockDir(transport, name, file_modebits=, dir_modebits=)

My proposal is to change this to:

   transport.get_lockdir(name, file_modebits=, dir_modebits=)

thus allowing the individual transport to choose which LockDir
implementation(s) they support.

Any thoughts?

Thanks
sRp