Upgrade to Tokio 1.0

[jaboatman]

• Upgrade to tokio 1.0
• Fix building the library outside of the directory (see below)
• Reduced number of futures based dependencies by directly depending on the required sub-crates.

For a long while I have been using mobc 0.5 because mobc 0.6 would always fail to compile with this error:

However, the crate builds fine when you clone it and build. So what's the deal? Turns out if you have a crate as a dependency and a dev-dependency, it will include the features for the crate from both builds. See here: rust-lang/cargo#7915

Since dev-dependencies aren't included when using the crate as a library, we were left with only the features specified in the [dependencies] section.

Well, starting in tokio 0.3 (and 1.0 for that matter) calling Runtime::new directly requires the rt-multi-thread feature:

https://docs.rs/tokio/1.0.1/tokio/runtime/struct.Runtime.html#method.new

This issue is fixed in this PR by adding the required feature.

Closes: #48

[importcjj]

Sorry for the late. At first, thanks for your contribution, I will publish a new version if there are no other problems.