Consider adding logging infrastructure to devicemapper-rs

[bmr-cymru]

There are situations where it would be useful to have more insight into the low-level operations being carried out in devicemapper-rs. This is especially the case with the work to implement udev cookie semaphore support where it is often necessary to correlate events in the test (and the cookie values they generate) with the system udev logs.

While it's possible to get so far with println!() it would be very helpful to at least have the option of enabling straightforward, timestamped logs for the critical interactions with other subsystems.

This should follow the log pattern for a library crate permitting users of the crate to configure whatever concrete logger is needed.

[mulkieran]

This can be considered to be in progress, in connection with #774