Remove `renew_lease` from our storage protocol

Fixes #224 (closed) Fixes #230 (closed)

Started off trying to accommodate Tahoe-LAFS <1.16 and >=1.16 with conditionals, morphed into just deleting renew_lease after #230 (closed) became apparent.

Apart from deleted code this includes some other fixes for lease-related tests to handle implementation changes in Tahoe-LAFS around telling time.

Note the merge target is #229 and this should fix most of the test failures with the new CI configuration supplied there.