This commit adds support to core resources to enable deferred deletion
of resources. Clients may optionally specify a time period after which
resources must be deleted via an object sent with their DELETE. That
object may define an optional grace period in seconds, or allow the
default "preferred" value for a resource to be used. Once the object
is marked as pending deletion, the deletionTimestamp field will be set
and an etcd TTL will be in place.
Clients should assume resources that have deletionTimestamp set will
be deleted at some point in the future. Other changes will come later
to enable graceful deletion on a per resource basis.
In order to support graceful deletion, the resource object will
need access to the TTL value in etcd. Also, in the future we
may want to get the creation index (distinct from modifiedindex)
and expose it to clients. Change EtcdResourceVersioner to be
more type specific (objects vs lists) and provide a default
implementation that relies on the internal API convention.
Also, rename etcd_tools.go to etcd_helper.go and split a few
things up.
