Add option to run server with a threadpool

[jaemk]

issue #167

• Add crates: threadpool, num_cpus
• Add start_server_with_pool function that mirrors the existing start_server function
• Add an Executor instance to the Server to spawn server processes in either a std::thread or a threadpool::ThreadPool
• Change Server::run signature to take a reference instead of ownership so it mirrors Server::poll and can be chained after a call to Server::pool_size

[jaemk]

Some test results (numbers are req/s) to confirm the default mode isn't affected:

---

cargo run --release --example hello-world
wrk -t4 -c100 -d10s http://localhost:8000

current	new - thread	new - threadpool
25500	25200	102200

---

cargo run --release --example database
# insert some data
curl -X POST -d 'some data' -H 'Content-Type: text/plain' http://locahost:8000/note
wrk -t4 -c100 -d10s http://localhost:8000/notes

current	new - thread	new - threadpool
6850	6820	8100

[tomaka]

The code looks good. Please also add an entry to CHANGELOG.md.

[jaemk]

Changelog is updated!