During pg_dump startup, acquire table locks in batches.

During pg_dump startup, acquire table locks in batches.

Combine multiple LOCK TABLE commands to reduce the number of
round trips to the server.  This is particularly helpful when
dumping from a remote server, but it seems useful even without
that.  In particular, shortening the time from seeing a table
in pg_class to acquiring lock on it reduces the window for
trouble from concurrent DDL.

Aleksander Alekseev, reviewed by Fabrízio de Royes Mello,
Gilles Darold, and Andres Freund

Discussion: https://postgr.es/m/CAJ7c6TO4z1+OBa-R+fC8FnaUgbEWJUf2Kq=nRngTW5EXtKru2g@mail.gmail.com