[varnish] Fix varnish plugin for varnishstat output

[codeurimpulsif]

Goal

• Fix json values fetch
• Remove old or deprecated fields
• Fix renamed fields

Informations

• Fix get_values() json (now values are in counters)
• Remove SMF
• Remove SMS
• Remove backend_toolate (deprecated)
• Replace sess_drop by sess_dropped
• Replace sess_pipe_overflow by sc_pipe_overflow