Clean up tracing ring buffer, add gdb command to inspect it

Clean up tracing ring buffer, add gdb command to inspect it

Summary:
- Restructure RingBufferEntry a little to clarify which fields are used
  by which entry types.
- Add a Generic entry type that's similar to Msg, but only
  supports zero-terminated, static strings, and has a full 64-bit data field.
- Add the rbprint gdb command, which is a loose copy of dumpRingBuffer().
- Add a gdb pretty-printer for SrcKey.

Reviewed By: mxw

Differential Revision: D6697081

fbshipit-source-id: 44eb4be38f796e8421f4d5fe18f3376a72451755