Ability to get partial result from the API

* Small memory devices seem to have issues with large JSON blobs.
This patch allows clients to request a portion of the text by
providing an offset and a maximum length.

* All "text" strings are virtually joined together, and only the
range after offset and no longer than maxlen is actually returned.

* A "continue-offset" value is set if there is more data.

Change-Id: Ib71800c1afa057bf87fd652ecb650bc38e33df68
Merge "Ability to get partial result from the API"