fix __lookupHost__ to invaidate helpers if host changes.

A few key points:

1. You can set a dynamic template's lookup host by calling
dt._setLookupHost(obj).

2. lookup hosts can have helpers that will be looked up during normal
Blaze.View lookup.

3. If a dynamic template's lookup host changes, all helpers will be
rerun. In other words, helpers create a reactive dependency on the
lookup host reactive variable.

4. You can find a lookup host helper by calling
DynamicTemplate.findLookupHostWithHelper(view, helperKey)