* 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
database feature now comes first. this ensures that the mmfiles collector thread
(owned by the mmfiles logfile manager) can always access the list of databases,
and that this list is not destroyed while the collector thread is still running.
Andrey Abramov
Wilfried Goesgens
Simon
Frank Celler
Jan
Vasiliy
jsteemann