test: use h2 shell instead of gsql for queries

Previously, gerrit gsql was used with the "Gerrit Code Review"
superuser to set up the ssh keys for the test user.  This stopped
working in gerrit 2.6.

Directly access the h2 database instead.
test: fixes for gerrit 2.6 behavior changes

type and description attributes in stream-events changed.

A new prefix now appears in retrieved comments
(e.g. Patch Set 2: Code-Review-1).
doc: fixed bad formatting around git_work_tree
doc: added COMPATIBILITY section

Notes about the compatible Gerrit versions.