Generate AppMaps of AppMap

[lachrist]

Tweaks I performed while appmapping appmap. The two main changes are performance related:

• Regularly compile and flush appmaps from the backend. Previously, all the messages were kept on the backend and compiled all at once after the test command returned. This caused memory problems on largish test suites. Now, every second, we check whether the backend has a track (recording array of messages) ready to be compiled and written to the disk.
• Support exclusion at the function level on the frontend. This makes instrumentation slower but prevent sending some useless data to the backend.

[lachrist]

I split this in two