Update 'executor' references to use std::shared_ptr<executor_type>

[jbaldwin]

Currently a good portion of the lifetimes around types that take a reference to an executor type (like thread_pool or io_scheduler) do so by reference. Its generally assumed the executor will outlive all of its 'child' like task types that are driven by it, but this is not guaranteed. The general cost of moving to a shared ptr should be cheap enough to make sure the lifetime issues do not occur?

I think prototyping it out and the benchmarking would be an appropriate approach, short lived tcp_client's could possibly impose a penalty but its probably worth it safety wise.