Implementation of "standby cluster" described in #657. Standby cluster consists
of a "standby leader", that replicates from a "remote master" (which is not a
part of current patroni cluster and can be anywhere), and cascade replicas,
that replicate from the corresponding standby leader. "Standby leader" behaves
pretty much like a regular leader, which means that it holds a leader lock in
DSC, in case if disappears there will be an election of a new "standby
leader".
One can define such a cluster using the section "standby_cluster" in patroni
config file. This section provides parameters for standby cluster, that will be
applied only once during bootstrap and can be changed only through DSC.
Add a field to an api to figure out if a master is there from patroni point
of view. It can be useful, when you have an alert, based on Auto Scaling
Groups, and then ASG decided to shutdown the current master, spin up a
new instance but the current master shutdown is stuck. In this situation
the current master is no longer a part of ASG, but patroni and Postgres
are still alive on the instance, which means a new replica will not be
promoted yet - this will lead to a false alert, saying that your cluster
doesn't have any master node.
