Use HSTORE instead of custom tables

Attempted to simplify the schema and the etl logic by using HSTORE for
collectd meta data.  Also take advantage of the postgres JSON functions
to convert collectd's write_http plugin's JSON output to Postgres
arrays and HSTORE.
Add additional indexes and checks to tables

This is part of the ETL process.  Additional CHECK constraints will
benefit the planner when constraint_exclusion is enabled.  Functional
indexes are added to the postgres plugin tables to help search by meta
data.
Update docs and examples