Port limits core API to API-v3 Part 2

This adds and adapts limits API to v3 tree using the new extension framework

Also changed the unit tests were done in v2 to look more like servers and other
API entry points tests.

Partially implements bp v3-api-core-as-extensions

Change-Id: I370e00bda0f8b328c5bd41bb3701ef77fc1d21d1
Merge "Port limits core API to API-v3 Part 2"