Whenever the snmp plugin was configured with a table with automatic field
discovery, if one of those fields was explicitly overridden in the config and
the value of is_tag was changed, the field would be duplicated, once as a tag
& once as a field.

This change fixes the behavior so that if a field is explicitly configured,
automatic table field discovery doesn't touch it.

Previously it was impossible to parse syslog timestamps without the date
being reported as year 0, due to the year not being specified

This reverts commit 697d8cea.