debuginfod: populate _r_seekable on request

debuginfod: populate _r_seekable on request

Since the schema change adding _r_seekable was done in a backward
compatible way, seekable archives that were previously scanned will not
be in _r_seekable.  Whenever an archive is going to be extracted to
satisfy a request, check if it is seekable.  If so, populate _r_seekable
while extracting it so that future requests use the optimized path.

The next time that BUILDIDS is bumped, all archives will be checked at
scan time.  At that point, checking again will be unnecessary and this
commit (including the test case modification) can be reverted.

Signed-off-by: Omar Sandoval <osandov@fb.com>