Add pool refreshing events

[jackfirth]

When a pool lease is disposed, a function could take the value and return a refresh event that does two things:

• Perform any necessary "refreshing" cleanup logic on the value that must happen before it may be reused, such as closing open database transations on a database connection
• Return a boolean as its synchronization result where a false result indicates that the value can't be reused, such as a connection that's been forcibly terminated

A single function of type (-> any/c (evt/c boolean?)) applied to the leased value can handle both use cases.

Obsoletes #70