Generic time series data model and TSDB writers

[rajatparida86]

ZMON generic time series data model

Impact

Provides a framework to optionally write Time series metrics from ZMON to other time series databases apart from KairosDB.

Deliverables

• Implement a generic time series data model for ZMON time series metrics
• Move ZMON time series metrics data model out of KairosDB writer and store code
• Implement TSDB writers (M3DB, IronDB, InfluxDB) and store