Changed protocol spec to make SETCONFIG only store it persistently when run
during INITREMOTE. I see no reason to support storing it persistently at
other times, and doing so would unnecessarily complicate the code.
Also, letting that be done would probably result in use for storing data that
doesn't really belong there, and special remote authors who don't
understand how the union merging works would probably be surprised the
results.
That complicated special remote programs, because they had to avoid making
PREPARE fail if some configuration is missing, because the remote might not
be initialized yet. Instead, complicate git-annex slightly by only sending
PREPARE immediately before some other request other than INITREMOTE (or
PREPARE of course).
This is mostly straightforward, but did turn out quite nicely stronly
typed, and with a quite nice automatic tokenization and parsing of received
messages.
Made a few minor changes to the protocol to clear up ambiguities and make
it easier to parse. Note particularly that setting remote configuration
is moved to a separate command, which allows a remote to set arbitrary data.
More complicated, but less asynchronous, which will make it easier for
special remote programs to use it, at the expense of some added complexity
in git-annex.
