vault: recover from standby losing etcd lease (#3031) (#3511)
vault: recover from standby losing etcd lease (#3031) (#3511)

This change makes these errors transient instead of permanent:

[ERROR] core: failed to acquire lock: error=etcdserver: requested lease not found

After this change, there can still be one of these errors when a
standby vault that lost its lease tries to become leader, but on the
next lock acquisition attempt a new session will be created. With this
new session, the standby will be able to become the leader.
rnaveiras/vault,hashicorp/vault,Aloomaio/vault,joelthompson/vault,rnaveiras/vault,quixoten/vault,mgk/vault,kingland/vault,kingland/vault,quixoten/vault,hartsock/vault,rnaveiras/vault,mgk/vault,joelthompson/vault,rnaveiras/vault,kingland/vault,Aloomaio/vault,hashicorp/vault,joelthompson/vault,naunga/vault,lemondrank/vault,mgk/vault,quixoten/vault,hartsock/vault,quixoten/vault,kingland/vault,hartsock/vault,joelthompson/vault,mgk/vault,hashicorp/vault,hashicorp/vault,naunga/vault,hashicorp/vault,hartsock/vault,kingland/vault,hashicorp/vault,hartsock/vault,rnaveiras/vault,lemondrank/vault,Aloomaio/vault,Aloomaio/vault