[GENERAL] pgbench support dynamic identifier name

[德哥]

HI,
   pgbench can support literal parameter now, so if need different pgbench 
client thread to support different table name.
   must use multi script , and set different name in the file. 
   but ,if pgbench can support it , will much better like sysbench.
   for exp:
```
vi test.sql
\set id random(1, 10000)
\set tables 10
\set suffix :client_id % :tables
select * from test:suffix where id=:id;


it can use pgbench -M prepared to support different tablenames.
```




best regards.



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