HPSS branch has landed!

[Andrew Bennetts]

The "HPSS" (High-Performance Smart Server) branch has landed.  The following
smart verbs have been implemented:

  * Branch.get_config_file
  * Branch.last_revision_info
  * Branch.lock_write
  * Branch.revision_history
  * Branch.set_last_revision
  * Branch.unlock
  * BzrDir.find_repository
  * BzrDirFormat.initialize
  * BzrDir.open
  * BzrDir.open_branch
  * Repository.gather_stats
  * Repository.get_revision_graph
  * Repository.has_revision
  * Repository.is_shared
  * Repository.lock_write
  * Repository.unlock
  * Transport.is_readonly

These are in addition to the existing VFS verbs, which the client will fall back
on for operations that don't yet have a better verb.

This means that clients using bzr+ssh:// and bzr:// in bzr.dev (and 0.16) will
*not* work with bzr 0.15 and earlier servers.  0.15 and earlier clients will
continue to with with bzr.dev servers.

Work has already started on further improvements:

  * Martin has a bundle that adds a Repository.tarball verb for speeding up
    an initial "bzr branch".
  * Robert is working on the next version of the wire protocol:
    https://code.launchpad.net/~bzr/bzr/hpss-protocol2
  * Robert has also started working on efficient graph delta operations:
    https://code.launchpad.net/~lifeless/bzr/graphdelta

Please try out bzr.dev with bzr+ssh:// and bzr:// and report any bugs you see!
We want to make sure they work at least as well as other transports (e.g. sftp),
and hopefully better!  So far only a few operations have been directly
optimised, but none should be worse.

Thanks very much to everyone for their help, especially in reviewing the large
volume of code involved.  Thanks especially to Robert, who has been immensely
helpful as always.

Making significant improvements in network performance should be *much* easier
from here.

-Andrew.