gc: improve box.info.gc output

Report vclocks in addition to signatures. When box.info.gc was first
introduced we used signatures in gc. Now we use vclocks so there's no
reason not to report them. This is consistent with box.info output
(there's vclock and signature).

Report the vclock and signature of the oldest WAL row available on the
instance under box.info.gc().vclock. Without this information the user
would have to figure it out by looking at box.info.gc().consumers.