Add TLI number to name of files generated by pg_waldump --save-fullpage

Hi all,
(Fujii-san and David in CC.)

Fujii-san has reported on Twitter that we had better add the TLI
number to what pg_waldump --save-fullpage generates for the file names
of the blocks, as it could be possible that we overwrite some blocks.
This information can be added thanks to ws_tli, that tracks the TLI of
the opened segment.

Attached is a patch to fix this issue, adding an open item assigned to
me. The file format is documented in the TAP test and the docs, the
two only places that would need a refresh.

Thoughts or comments?
--
Michael