error context for vacuum to include block number

On Wed, Aug 07, 2019 at 04:51:54PM -0700, Andres Freund wrote:
https://www.postgresql.org/message-id/20190807235154.erbmr4o4bo6vgnjv%40alap3.anarazel.de
| Ugh :(
| 
| We really need to add a error context to vacuumlazy that shows which
| block is being processed.

I eeked out a minimal patch.

I renamed "StringInfoData buf", since it wasn't nice to mask it by
"Buffer buf".

postgres=# SET statement_timeout=99;vacuum t;
SET
2019-11-20 14:52:49.521 CST [6319] ERROR:  canceling statement due to statement timeout
2019-11-20 14:52:49.521 CST [6319] CONTEXT:  block 596
2019-11-20 14:52:49.521 CST [6319] STATEMENT:  vacuum t;

Justin