pgbench support dynamic identifier name

[德哥 <digoal@xxxxxxx>]

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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