Fix the interaction between leases and mutable writes

Mutable writes are performed using slot_testv_and_readv_and_writev. The implementation of this method also renews leases.

If every write implies a lease renewal then writing becomes very expensive for users or charging a fair price becomes very hard for storage providers.

Everything is simpler if writes and leases aren't linked like this. Change the plugin storage protocol so that mutable writes have nothing to do with leases.