Deny writes without active lease

Fixes: #21 (closed)

This is related to #47 (closed) as well since slot_testv_and_readv_and_writev is the only "write" operation in the API and there is no way to acquire a lease on a mutable share without writing the mutable share. So the changes here do some pass validation in that method - but only enough to implement #21 (closed). #47 (closed) will receive more attention in a follow-up PR.