…check fails

When SchemaQueries.newInstance() throws or schema agreement check fails,
the cleanup code (resetting currentSchemaRefresh, completing
firstSchemaRefreshFuture, draining queuedSchemaRefresh) was never
executed, permanently blocking all future schema refreshes for the
session.

Extract cleanup into onSchemaRefreshComplete() and call it from all
three completion paths: agreement error, newInstance() exception, and
normal completion.

Fixes: #841

When the control connection endpoint doesn't match any node in metadata
(e.g. with PrivateLink or proxy setups), fall back to the first available
node instead of immediately throwing an exception.