Feature/disk cache sync on change and sled connection config

[BaxHugh]

This PR concerns only the DiskCache

Closes https://github.com/jaemk/cached/issues/190

The main features of this PR are

• test improvements in disk.rs: PR for this is https://github.com/jaemk/cached/pull/193 (or just within my clone for the sake of isolating changes: https://github.com/BaxHugh/cached/pull/4)
• feature to write changes to disk every time a cache is changed (value set, removed or refreshed) - PR within my clone to give isolated diffs: https://github.com/BaxHugh/cached/pull/2
• feature to provide a sled::Config to create the connection from + connection() and connection_mut() functions to give users more control over the sled backend. - PR within my clone to give isolated diffs: https://github.com/BaxHugh/cached/pull/3

This replaces https://github.com/jaemk/cached/pull/192

(the PRs in my clone are all stacked, but I don't think I can do that within this repo, so it is in one big PR to master here)

[BaxHugh]

This will have a few small merge conflicts with the changes in https://github.com/jaemk/cached/pull/185. So happy to merge this after that PR is merged.

But if someone could have a little review @omid @jaemk, that would be fantastic as this is something I'm hoping to use very soon in my work project. (I've linked to separate smaller PRs to make diffs easier to digest).

Also, if there's any PRs in particularly that it'd be helpful for me to help with reviewing, let me know..

[jaemk]

@BaxHugh would you mind resolving the disk.rs conflicts for me

[BaxHugh]

@jaemk, no problem. Done: (rebased onto master)