Add option to allow a delay for the initial GC collection

Add an option that allows the initial run of the GC daemon to be delay by some
number of seconds. This provides a convenient way for the GC daemon to be run
on multiple nodes without overlapping the collection and cleanup effort among
nodes. The option is called initial_gc_delay and is specified in seconds. The
value only applies to the initial collection run when the riak_cs node is
started.