[Feat]: support keeping dbengine Tier0 in memory only

[FedericoCeratto]

Problem

On embedded systems disk writes might be a concern due to memory wear. Netdata currently provide a choice between dbengine or memory-only storage, while a hybrid solution could provide a good balance.

Description

Support keeping dbengine's Tier0 datapoints in RAM (never writing them to disk), while Tier1 and higher are written to disk as usual.
In case of restart Netdata should be able to handle the absence of datapoints in Tier0.

Importance

nice to have

Value proposition

This provides a balance between writing every datapoint on disk or writing no data at all.
E.g. if the decimation ratio between Tier0 and Tier1 is 60 this would decrease disk write traffic by 60 times.

Proposed implementation

No response