Add an assocations-like "Edges" framework

Summary:
We have a lot of cases where we store object relationships, but it's all kind of messy and custom. Some particular problems:

• We go to great lengths to enforce order stability in Differential revisions, but the implementation is complex and inelegant.
• Some relationships are stored on-object, so we can't pull the inverses easily. For example, Maniphest shows child tasks but not parent tasks.
• I want to add more of these and don't want to continue building custom stuff.
• UIs like the "attach stuff to other stuff" UI need custom branches for each object type.
• Stuff like "allow commits to close tasks" is notrivial because of nonstandard metadata storage.

Provide an association-like "edge" framework to fix these problems. This is nearly identical to associations, with a few differences:

• I put edge metadata in a separate table and don't load it by default, to keep edge rows small and allow large metadata if necessary. The on-edge metadata seemed to get abused a lot at Facebook.
• I put a 'seq' column on the edges to ensure they have an explicit, stable ordering within a source and type.

This isn't actually used anywhere yet, but my first target is attaching commits to tasks for T904.

Test Plan: Made a mock page that used Editor and Query. Verified adding and removing edges, overwriting edges, writing and loading edge data, sequence number generation.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, 20after4

Differential Revision: https://secure.phabricator.com/D2088