First-class support for leases

[alecthomas]

Leases are super useful for all kinds of things.

// Define a lease namespace.
var userUpdateLease = ftl.Lease("user", "update")

//ftl:export
func UpdateUser(ctx context.Context, req UpdateUserRequest) error {
  // Obtain a lease on a single user.
  defer userLease.Acquire(ctx, req.UserId).Release()
  // Do a bunch of stuff.
}

The lease key should be a hierarchy perhaps? Should acquiring a lease on user acquire it for all children? Should we even do that?

func Lease(key...string) LeaseHandle

type LeaseHandle struct {}
func (LeaseHandle) Acquire(ctx context.context, key...string) LeaseReservation

type LeaseReservation struct {}
func (LeaseReservation) Release()

Leases would need a table, and a reaper job to cleanup expired leases. We are using a similar concept already for runner and controller reservations which we could probably rewrite using these.

Or should the API be more like:

func Lease(ctx context.Context, key... string) LeaseReservation

//ftl:export
func UpdateUser(ctx context.Context, req UpdateUserRequest) error {
  // Obtain a lease on a single user.
  defer ftl.Lease(ctx, "update", "user", req.UserId).Release()
  // Do a bunch of stuff.
}

Will probably need an additional method on VerbService for renewing the lease.