Add storage for repository symbol tracking

Summary: See T315 for an extensive description of this feature. Adds the
descibed storage table.

Test Plan: Used phpsh to read/write symbol objects.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, epriestley, tuomaspelkonen

Differential Revision: 897