Only evaluate expressions when there is a sink for the log message

[stefandraskovits-aaa]

Creating log messages can be expansive (e.g. network response body mapping to string).
Currently the expressions for messages (on the caller sites) are already generated when calling the log message.

When wrapping the expression in a closure the closure can only be evaluated if there is a sink even consuming the the message.

with @autoclosure this doesn't change the way of interacting with logbook