Fix lifetimes

[elihunter173]

What did you implement:

After a dance with borrowck, I updated the lifetimes of problematic methods in lib.rs where the return's lifetime was unnecessarily tied to some of the inputs. In particular, {Containers, Images, Networks, Services, Volumes}::get() now return a {Container, Image, Network, Service, Volume} which is tied to the lifetime of the Docker they container rather than the lifetime of the {Containers, ...} which created them. This means we no longer have to create unnecessary locals such as images in the following code to avoid the compiler complaining about temporary value dropped while borrowed

let images = docker.images();
let mut stream = images.build(&options);

Instead, we can write the following as you would intuitively expect to work.

let mut stream = docker.images().build(&options);

Various other methods were also fixed. Generally, anything that returned a stream is now exclusively tied to the lifetime of the Docker used to create them (or Transport for transport.rs) rather than the lifetime of both the Docker and whatever they use as input (e.g. ExecOptions for Container::exec()).

Further, I renamed lifetimes from 'a to 'docker and made some lifetimes explicit to make the code easier to read and understand.

Closes: #271

How did you verify your change:

Ran unit tests.

What (if anything) would need to be called out in the CHANGELOG for the next release:

This should be backwards compatible because in every case I made the lifetimes less restrictive. Nevertheless, I updated the changelog to mention it.

[thomaseizinger]

Thank you!

This also fixes a bug where it was impossible to return the Stream returned from logs() due to lifetime issues!