RAS plugin - fix for too many open files handlers

[p-zak]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

Details

Database driver doesn't close file handler properly when db.Close() is called.
This may lead to too many open files for Telegraf process (returned by OS).

In this fix sql.Open() is called once in Start() and db.Close() is also called only once in Stop().

This is also optimization (open DB just once, not in every Gather()) and following along guide from comment for func (db *DB) Close() error in sql.go:

// It is rare to Close a DB, as the DB handle is meant to be
// long-lived and shared between many goroutines.

The same approach was introduced in inputs/postgresql plugin.