The DGC abstraction is used for the server side of the distributed
garbage collection algorithm. This interface contains the two
methods: dirty and clean. A dirty call is made when a remote
reference is unmarshaled in a client (the client is indicated by
its VMID). A corresponding clean call is made when no more
references to the remote reference exist in the client. A failed
dirty call must schedule a strong clean call so that the call's
sequence number can be retained in order to detect future calls
received out of order by the distributed garbage collector.

A reference to a remote object is leased for a period of time by
the client holding the reference. The lease period starts when the
dirty call is received. It is the client's responsibility to renew
the leases, by making additional dirty calls, on the remote
references it holds before such leases expire. If the client does
not renew the lease before it expires, the distributed garbage
collector assumes that the remote object is no longer referenced by
that client.

A lease contains a unique VM identifier and a lease duration. A
Lease object is used to request and grant leases to remote object
references.

A VMID is a identifier that is unique across all Java virtual
machines.  VMIDs are used by the distributed garbage collector
to identify client VMs.