* 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
* issue 535.1: ensure recovery success if link recovery starts right at the previous marker
* backport: initialize members
* backport: use string_ref instead if string copies
* issue 526.3: update analyzer feature to store analyzer definitions in per-vocbase system collections
* address merge issues
* address another merge issue
* issue 511.8: remove a redundant IResearchLinkDrop marker, it does the exact same thing as IndexDrop that is also written to the WAL
* add a comment for deprecated markers
* issue 511.4: ensure IResearchLink can be initialized without view as required for db-server, minor code fixes and cleanup
* remove unnessesary checks
* revert last change due to test failures
* fix typo in change revert
* try to address random test failures
* 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
* issue 373.2: move toVelocyPack into LogicalDataSource
* backport: move static DataSource related strings into StaticStrings, add support for registering snapshots on DBServer views
* Changes since last PR: use LogicalDataSource for Methods::StateRegistrationCallback instead of TRI_voc_cid_t to avoid unnecessary lookups
* backport: address cluster LogicalDataSource resolution failure