pgbench support dynamic identifier name

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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