Improve subscriber lease extension logic

[dbolduc]

The current implementation is to extend all leases every N seconds. It is an ok strategy. The customers aren't charged for these RPCs. It could be a lot better though, and has potential to clog the lease event loop before we have #3975.

We should:

• keep in the LeaseInfo the last time we attempted to extend a lease. (or None, for new messages).
• only extend leases for messages where 80% of the current lease has been used.

This looks like modifying the Leases::retain functions. And probably updating MessageStream / LeaseState tests that assume a certain behavior.