Bazaar dirstate locking (was: lock_* API change)

[Robert Collins]

Data point: the main reason commits take minutes is because we're
manipulating a non-trivial-size database using VFS operations, over
high (> 1ms) latency, low bandwidth links.

Karl Fogel is working on Savannah to rectify this. Until its fixed, I
*totally expect* operations to be slow and take ages. Networked
databases with client side manipulation of the disk data structure
*take time*: git http pulls are very slow, for precisely the same
reason.

-Rob