[server] use a connection instead of a session for user authentication

Use repoapi instead of dbapi for cwctl shell, upgrade and db-init Hopefully nobody uses dbapi-specific API in their migration scripts. I guess we'll find out.

[server/repo] use internal_cnx in init_sources_from_database

[server/session] avoid spurious warnings from Session.close Add internal non-noisy _rollback method.

[server] rename session to cnx in querier and plan They don't actually need a session, so let's make that clear.

[repo] Use a connection instead of a session for repo.stats()

[repo] Use a connection instead of a session for repo.gc_stats()

[repo] Use a connection instead of a session for repo.get_versions()

[webconfig] get_repository moved to repoapi

[schemaserial] Replace 'cursor' with 'cnx' We don't have cursors in rql, since execute is synchronous.

[book] basic documentation for LDAPfeed

[testlib] Use session.sessionid instead of deprecated session.id

Add data/pgdb to hgignore patterns PostgresTestDataBaseHandler uses it as datadir.

[test] make unittest_postgres pass Use PostgresApptestConfiguration which starts a postgresql cluster locally for the duration of the test.