PostgreSQL directory import from internal -> internal causes hang if overwrite is enabled

Repro steps (using Crowd 2.6.4 set up on PostgreSQL 8.4.17):

1. Create an internal directory dirA
2. Add 2 groups to dirA:

• users-group
• admins-group

1. Add two users to dirA

• user1 (member of users-group and admins-group)
• user2 (member of users-group)

1. Create an internal directory dirB
2. Import users from dirA to dirB, without overwriting existing

• 2 users, 2 groups, 3 group-memberships imported

1. Import users from dirA to dirB, this time overwriting existing

• Import hangs on org.postgresql.core.v3.QueryExecutorImpl.processResults() (in PostgreSQL JDBC driver)

Found while investigating CWD-1739.