Remove fibers for Meteor vNext

[afrokick]

To support Meteor 3.0 we need to remove fibers and adjust another features.

[zodern]

This month @leonardoventurini and I are focused on getting tracing to work on Meteor 3. I think the other parts of the agent will be relatively easy to make work without fibers. The plan is to keep support for Meteor 1.4 even with adding support for Meteor 3, which might complicate things.