agent/caching: use locks on LeaseCache.Send (#6374)
agent/caching: use locks on LeaseCache.Send (#6374)

* agent/caching: use locks on LeaseCache.Send to prevent running multiple renewer goroutines

* update comment

kingland/vault,hartsock/vault,joelthompson/vault,hashicorp/vault,joelthompson/vault,joelthompson/vault,joelthompson/vault,hashicorp/vault,rnaveiras/vault,hartsock/vault,rnaveiras/vault,kingland/vault,rnaveiras/vault,hashicorp/vault,hartsock/vault,quixoten/vault,quixoten/vault,quixoten/vault,rnaveiras/vault,hashicorp/vault,quixoten/vault,hartsock/vault,rnaveiras/vault,hashicorp/vault,kingland/vault,hashicorp/vault,kingland/vault,kingland/vault,hartsock/vault