* allow to access last operation tick of the transaction
* modify IResearchLink to rely on last commtted tick
* get rid of writing arangosearch markers to WAL
* further implementation
* local changes in iresearch
* add recovery states
* properly handle link creation during recovery
* adjust test cases
* properly handle nested transactions
* ungreylist recovery tests
* add more recovery tests
* do not use transaction to pass recovery tick
* do not store recoveryTick in MMFilesRecoveryState
* adjust tests
* fix mmfiles
* cleanup
* add context validity check
* fix tests
* fix more tests
* ensure subscription is not being released during commit
* address review comment
* final cleanup
* fix crash
* fix tests
* address test failures
* address review comments
* address review comments
* properly set recovery tick even if no recovery happened
* start maintenance arangosearch threads after recovery is done
* ensure flush subscription is not being deallocated while in use
* add some tests
* properly determine storage engine
* adjust default view options
* stick to old index meta format in 3.5
* address test failures
* and cluster tests
* issue 496.3: move more coordinator-related logic out of TRI_vocbase_t, rename some arangosearch view configuration parameters, remove some consolidation policies, update iresearch to revision 6fd9760d81b136f769e277ea5b8f53996ed7a1ca
* address potential deadlock between link creation and FlushThread
* remove code causing nullptr access
* add back lock around reader reopen
* revert: address potential deadlock between link creation and FlushThread
* invalidate payload for each field in FieldIterator before setting a value
* update iresearch to codebase as of 20181011
* backport: address cluster test failures
* backport: address dump test failures
* backport: address discrepency in view creation between single-server and cluster
* backport: address test failure on cluster (revert change)
* backport: address test failures
* backport: address MSVC build issues
* backport: address issue with LogicalDatasource destructing after TRI_vocbase_t
* Revert "backport: address issue with LogicalDatasource destructing after TRI_vocbase_t"
This reverts commit 4f9880bbaa22194dfbb604b5a54658de1d447ac1.