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

src/docs/developer/using_edges.diviner

@@ -0,0 +1,31 @@
+@title Using Edges
+@group developer
+
+Guide to the Edges infrastructure.
+
+= Overview =
+
+Edges are a generic way of storing a relationship between two objects (like
+a Task and its attached files). If you are familiar with the Facebook
+associations framework, Phabricator Edges are substantially similar.
+
+An edge is defined by a source PHID (the edge origin), a destination PHID
+(the edge destination) and an edge type (which describes the relationship,
+like "is subscribed to" or "has attached file").
+
+Every edge is directional, and stored alongside the source object. Some edges
+are configured to automatically write an inverse edge, effectively building
+a bidirectional relationship. The strength of storing relationships like this
+is that they work when databases are partitioned or sharded.
+
+= Reading Edges =
+
+You can load edges with @{class:PhabricatorEdgeQuery}.
+
+= Writing Edges =
+
+You can edit edges with @{class:PhabricatorEdgeEditor}.
+
+= Edges and Lisk =
+
+@{class:PhabricatorLiskDAO} includes some builtin support for edges.