NXCALS-4052 Better performance on hierarchyService.getVariables(hierarchyId) API action

Merge request template

Get all variables by Hierarchy node id [was 3,8s | now 800ms]

Before

2020-07-26 05:45:12.891 [DEBUG] [main] HierarchyClient - [HierarchyClient#getVariables] ---> POST https://nxcals-service/hierarchies/10002308/variables HTTP/1.1
2020-07-26 05:45:15.842 [DEBUG] [main] HierarchyClient - [HierarchyClient#getVariables] <--- HTTP/1.1 200 (2950ms)
duration milis: 3087

After

2020-07-26 19:05:37.869 [DEBUG] [main] HierarchyClient - [HierarchyClient#getVariables] ---> POST https://nxcals-service/hierarchies/10002308/variables HTTP/1.1
2020-07-26 19:05:38.539 [DEBUG] [main] HierarchyClient - [HierarchyClient#getVariables] <--- HTTP/1.1 200 (670ms)
duration milis: 797

COMPATIBILITY NOTE

Please make sure that this change is BACKWARDS COMPATIBLE and a new server will work correctly with clients using old APIs versions.

It may be that data sent in the new format will break the old clients.

In case of any potential incompatibility we have to inform our clients and schedule release & deployment with them.

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@acc-logging-team - please have a look for approval.

Please describe the merge request here.

Closes NXCALS-4052