Fix bug in replication mode and snapshot isolation mode.

Fix bug in replication mode and snapshot isolation mode.

In replication mode and snapshot isolation mode when a command fishes,
pgpool waits for a ready for query message but forgot that some
commands (for example SET ROLE) produces a parameter status
message. As a result pgpool errors out that other message arrives
before the ready for query message.  Deal with the case when a
parameter status message arrives.

Here is the test case written in pgproto data format.

'P'	""	"SET ROLE TO foo"
'B'	""	""	0	0	0
'E'	""	0
'P'	""	"SELECT 1"
'B'	""	""	0	0	0
'E'	""	0
'S'
'Y'

Backpatch-through: v4.1.