Concurrent tree-mapping (2.5.2)

[Rich Winkel]

Hi, maybe this is fixed in the current version, but in listening
to the disk drives on two rsyncing hosts in the same room, it was
apparent that the target machine only started mapping out its directory
structure after the source machine had finished doing so, doubling
the startup time.  Seems like concurrency would be worth adding if
it's not too much trouble.

Rich