Add method to parse parameter list in query string to folly::Uri

Add method to parse parameter list in query string to folly::Uri

Summary:
Add a method to folly::Uri to get parsed query string

e.g. http://localhost?key1=foo&key2=bar

We can get key value map containing:
"key1" => "foo"
"key2" => "bar"

Test Plan:
fbconfig folly/test
fbmake runtests_dbg

Reviewed By: tudorb@fb.com

Subscribers: wormhole-dev@

FB internal diff: D1455158

Tasks: 4768038