* issue 381.4.1: create a separate IResearchLink implementation for use with ClusterEngine
* backport: ensure StorageEngineMock assigns unique transaction ids
* delegate index normalization on the ClusterEngine to the 'actualEngine'
* add initial implementation of scatter view rule and node
* add tests for `IResearchViewNode` and `IResearchViewScatterNode`
* add missing check
* modify IResearch execution nodes to use references instead of pointers
* use view id in searialized `ExecutionNode` representation instead of the name
* add cluster mode stubs and checks
* very first attempt to distribute IResearchViewNode
* further implementation of cluster-wide arangosearch views
* fix invalid json format
* add tests for coordinator iresearch view
* allow to retrieve a list of existing views on a coordinator
* more tests for coordinator iresearch view
* some fixes to enable query explanation
* remove Collection dependency from RemoteNode
* remove unnecessary remote ArangoSearch view scatter
* fix explanation appearance
* add some assertions
* minor fixes
* implement IResearchViewCoordinator::updateProperties
* fix view DDL issues
* handle link modifications in DDL operations
* add coordinator implementation of iresearch view links
* fix tests
* further coordinator based view DDL implementation
* further IResearchViewCoordinator implementation
* add initial implementation of AgencyMock
* fix some tests
* code cleanup
* extend test + some fixes
* more tests for IResearchViewCoordinator
* fix tests for IResearchLinkCoordinator
* some fixes after merge
* fix tests
* remove declaration of nonexistent (previously removed) method
* some fixes after review
* remove string duplication
* more tests and fixes
* more fixes and tests
* more tests
* one more test
* fix 'use-after-free' asan error
* fix non-enterprise tests issues
Vasiliy
Andrey Abramov
Jan