pgbouncer - pgbouncer: Remove drop_on_error, it was a bad idea.

Log Message:
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Remove drop_on_error, it was a bad idea.

It was a workaround for broken plan-cache in Postgres,
but it's behviour for common case - some queries giving always errors -
is very nasty - it can drop all connections in pool.

Modified Files:
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    pgbouncer/doc:
        config.txt (r1.12 -> r1.13)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/doc/config.txt.diff?r1=1.12&r2=1.13)
    pgbouncer/include:
        bouncer.h (r1.18 -> r1.19)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/include/bouncer.h.diff?r1=1.18&r2=1.19)
    pgbouncer/src:
        main.c (r1.46 -> r1.47)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/main.c.diff?r1=1.46&r2=1.47)
        server.c (r1.30 -> r1.31)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgbouncer/pgbouncer/src/server.c.diff?r1=1.30&r2=1.31)