Separate Avatica from "core" Calcite build

Uber-JIRA-issue for tracking the separation of Avatica from the rest of the Calcite build.

Long term, Avatica can be a standalone project, but we're not there yet in terms of governance (enough people intimately involved). One thing we can do now is to separate Avatica from Calcite and begin "getting them used to" not being next to one another.

DISCUSS thread: https://mail-archives.apache.org/mod_mbox/calcite-dev/201601.mbox/%3C56ABCCFF.3010205%40gmail.com%3E