Fix worker starvation issue in ThreadPool

Signed-off-by: Amr Ali <amr@databracket.com>
Add __contains__ interface to Json mixin

Signed-off-by: Amr Ali <amr@databracket.com>
Fix construction infinite recursion and out-of-order initialization in protocol messages

Signed-off-by: Amr Ali <amr@databracket.com>