* 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
# Conflicts:
# arangod/IResearch/IResearchLink.cpp
* 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
Andrey Abramov
Jan Christoph Uhde
Vasiliy
Frank Celler
Simon
Jan