db: VersionEdit can understand the format introduced in PR 3488.

db: VersionEdit can understand the format introduced in PR 3488.

RocksDB's PR 3488 introduced a new format of `VersionEdit`
encoding which is not understandable for older versions.
Thus, the change broke forward compatibility and has been
reverted in PR 3762. Later, PR 3765 reiterated the concept
but in a way that does not provide compatibility with the very
short-living format of PR 3488.

This change tries to address the issue of accessing DBs
in format of 3488 by ignoring the special entries.

Fixes: http://tracker.ceph.com/issues/25146
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>