* 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
non-sealing of datafiles could actually happen when more than a single journal
was found at startup, and all but the last journals were converted into datafiles
automatically. these datafiles were not sealed, triggering the assertion failures
later in other places
* fix potential overflow in CRC marker check when a corrupted CRC marker is found at the very beginning of an MMFiles datafile
* added recovery test for marker corruption
* Make isRestore work in the cluster.
This covers sharded collections with default sharding and non-default
sharding.
* always use locally generate revision ids for storing and looking up documents