type issues Template DB PostgreSQL

I tried out the new PostgreSQL template and came across a few issues:

1. Discovered items "DB {#DBNAME}: Queries * time" should have type Numeric (float) and not Numeric (unsigned). 

2. Item "Replication: lag in seconds" should have type Numeric (float) and not Numeric (unsigned). 

3. At least when installing from the PGDG repos, pg_isready is not in the $PATH for the zabbix user, leading to false alerts. So perhaps something could be added to the README.md about that. 

I've also posted this as a comment in issue #ZBXNEXT-5257