Rewrite of ectr_report.

Split into each style and fold style reports. Callbacks are different for each.
GC is now an opaque object.
Rewrite of ectr_srv:
* Reports now run in a child process.
* GC is given as an init parameter
s/tid/tab/g.
Update ectr:start_link/3->4 after API changes.
Remove dead types.
Expose new_gc in the public api.
Export new_{each,fold}_report from API.
Version bump to 0.2.0.