Do not keep ptr to running async function, use runtime lookup instead

Summary: Do not update AsioContext::m_current everytime the currently executed
async function switches. Use runtime lookup instead.

Makes it easier to:

- switch running async functions from JIT
- unwind thru multiple async functions inside unwinder
- merge async stack trace into PHP stack trace

Makes it slower to retrieve async stack trace, but that cost will
disappear once we fully switch to unified stack trace. Hopefully the
extra cost is either not significant, or compensated by removed store
on the hot path.

Reviewed By: @jdelong

Differential Revision: D1365742