[shared-buffer] Implement Send + Sync

- Implement Send + Sync for SharedBuffer, SharedBufferSlice, and
  SharedBufferSliceMut

Test: No tests since this is a memory safety property. Memory safety
      argument provided in comment.

SEC-84

Change-Id: I221837958dd6b2d79f8dab460af7a826175d5eaf
1 file changed