create Change.fronChdict and move instance creation out of db.changes

[dustin]

The BuildRequest and SourceStamp classes now have from**dict class methods. This provides a nice separation between the db code (which merely produces a dictionary representing a particular database row) and the process classes.

I'd like to do the same for the Change and the db.changes class.

[Dustin J. Mitchell]

Update change sources to use the new addChange parameter names

Note that sendchange will still use the old parameter names on the wire, although the master accepts the new names now. Once 0.8.4 or higher are the majority of installed masters, we can alter sendchange.

Refs #1905.

Changeset: 56d87cb6f798a613da76990b039ece45aea6e3a8

[dustin]

So this is done, and thoroughly. There were several related commits that I pushed all at once, but not all of them showed up.