Add health_check_password directive.

Add health_check_password directive.
Enhance check_replication_time_lag() error message.
Patch contributed by Nicolas Thauvin.

His message:

We are currently not able to specify the password of the health check
user, which is needed at least for streaming replication lag.  So I
added the possibility to specify it in the configuration file, in the
attached patch.

Also, I find the error message in check_replication_time_lag()
(pool_worker_child.c) not friendly :

pool_error("check_replication_time_lag: DB node is valid but no
persistent connection");

What about sending this error message to the debug level and adding a
second message on the error level?  Fro example:

pool_error("check_replication_time_lag: could not connect to DB node %d,
check health check configuration", i);