chunkd near-term enhancements

[Jeff Garzik <jeff@garzik.org>]

Here are a few chunkd enhancements that are currently on my drawing 
board, for the near term:

(CHO_xxx denotes new chunkd network protocol commands, as listed in 
include/chunk_msg.h)

* CHO_SET_SERVERS:
chunkd shall maintain a per-connection buffer known as SERVER_LIST. 
This chunkd command is issued by the client prior to using a 
SERVER_LIST-related command (see below), to reset the contents of the 
connection's SERVER_LIST buffer.

* CHO_RCP:
copy a single object to each remote server in SERVER_LIST

* CHO_PUT_THRU:
like PUT, but causes chunkd to further replicate the incoming object to 
each remote server in SERVER_LIST

* CHO_APPEND:
append data onto an object.

* CHO_APPEND_THRU:
append data locally, and, replicate foreach remote server in SERVER_LIST

The authentication used in chunkd->chunkd connections is the logged-in 
username/shared-secret combination.

	Jeff