Introduce lower limit for gc() maxRevisionAge

Introduce and enforce a lower limit for maxRevisionAge in VersionGarbageCollector.gc().

OAK-4494 changes the way documents in a cache are considered up-to-date. In addition to the modCount value it also considers the modified timestamp. To work properly, a new document must have a modified timestamp that is different from a previous incarnation (i.e. before gc removed it). The version GC should therefore not remove documents with a maxRevisionAge less than the modified resolution (5 seconds).