[document] lease check activated (OAK-2739)

[Stefan Egli]

Hi all,

Just a quick heads-up: I¹ve activated a Œlease check¹ with OAK-2739 in
trunk: this checks upon every invocation of DocumentStore if the local lease
is still valid. If it is not, it means that the instance is misbehaving and
that others potentially have seen it as inactive. Thus the local instance
will automatically shutdown and not do any further writes towards
DocumentStore.

Cheers,
Stefan