perl plugin: More cleanups.

perl plugin: More cleanups.

*   Struct "metric_t": unnecessary fields "type", "plugin", "ds" have
    been removed. The field "value_ds_type" has been renamed to
    "value_type".
*   The reference counted, multiple instance "meta_data_list_head_t" type
    has been removed. Manually doing the reference counting all over the
    place is error prone and it seems like premature optimization.
*   The "<type>_destroy" functions must handle NULL gracefully. Removed
    NULL-checks around their invocation to improve readability.

# Conflicts:
#	src/daemon/metrics_list_test.c