Implement in-place inject-as-head

Depending on the outcome of [ARAKOON-455](http://jira.incubaid.com/browse/ARAKOON-455), this could supersede #313 (after merging this, 1.6 should be merged into 1.7).

If #313 is merged instead, this can be closed.

CI running (*arakoon-git-launcher* 185).