Introduce --dont-kill-backend option.

pg_repack needs to take an exclusive lock at the end of the
reorganization. If the lock cannot be taken after duration
--wait-timeout option specified and this option is true,
pg_repack gives up to repack a target table instead of
cancelling conflicting backend. False by default.

regress/sql/repack.sql

@@ -230,3 +230,8 @@ CREATE TABLE test_schema2.tbl2 (id INTEGER PRIMARY KEY);
 \! pg_repack --dbname=contrib_regression --schema=test_schema1 --table=tbl1
 -- => ERROR
 \! pg_repack --dbname=contrib_regression --all --schema=test_schema1
+
+--
+-- don't kill backend
+--
+\! pg_repack --dbname=contrib_regression --table=tbl_cluster --dont-kill-backend