The SNMPCollector is currently focused and oriented to send metrics to
InfluxDB, using internally specific functions of InfluxDB client to
manage and send those metrics.

This PR tries to break with this dependency on metric internal management
adopting the current telegraf metric definitions.
With this abstracion, a new engine is created in order to send those
generic metrics to specific backends in a centralized way.

The new metric sender engine also adopts the current telegraf buffer. The metrics
are now stored on the buffer instead of being written each time that
a measurement produces a small set of them.

The proposed engine is changed and it is based on the current telegraf
way: a ticker is defined and the metrics are flushed on several batches,
reducing drastically the number of request that are being done and with
the change to control the current buffer size and make operations on top
the output.

Since all the metrics are stored on a top-level buffer, the send process
is being done by the backend implementation. This PR redefines the
current influxdb to be a backend that only performs the connection, write
close.

The internal selfmon metrics -measurement, runtime, outdb stats- are
refactored to follow the same generic metric definition, using, as always,
the 'default' backend defined, now, as an Output.

With this behaviour, a new Kafka Backend has been created to be able to
write metrics to several brokers using JSON output format. The code is
copied/modified from the current telegraf.output Kafka

The SNMPCollector OutDB now is related with an Output instead of
InfluxDB Server. All the management of Outputs and new Kafka Server
can be done on the UI and via API. The output relation with the backend
is, right now 1:1, and a Device can have only 1 output attached

BREAKING CHANGE: the metrics are now stored on internal buffer and
written to the final backend based on time. The write process is being
done each FlushInterval and splitted into several request based on
the buffer length and the MetricsBatchSize.

BREAKING CHANGE: the InfluxDB UI component is being moved to a new
section and the relation with the SNMPDevice is break and changed to an
Output. The field BufferSize is being removed from API request and new
DB initializtion, but mantained on old configurations to perform a
migration process

fix #343