LRS-30 Support LRS 2.0

[milt]

LRS-30
Wherein we strive to be conformant per version "2.0" of the conformance tests:
https://github.com/adlnet/lrs-conformance-test-suite/tree/LRS-2.0

Will require a decent rework to handle both old and new versions, this branch starts that process.

Currently passes 2.0 branch tests in both CLJ and CLJS

Also see: yetanalytics/xapi-schema#73 for the xapi-schema-component

TODO:

• full dispatch on version (is custom now)
• Update for release version of tests
• Fix hack to return 1.0.3 on non-statement routes in interceptor
• properly destructure and use contextAgents and contextGroups in memory impl
• pass version to impl
• add/ensure support for as-yet untested Last-Modified header
• full backwards compat pending fixes to the 2.0 branch and fixes to master