Filter sessions on the DB side to check if key is available

See merge request !58

Daniel Gerhardt authored 7 years ago and Daniel Gerhardt committed 7 years ago

This should massively reduce the time needed for session creation.
Additionally, the `Transactional` annotation has been removed because
it did not have any effect in this case.

Log level for DB logging was overriden by the fallbacki value in some
cases.

ARSnova no longer supports JDK 7 which is still the default for
Travis CI Java builds. `sudo` is disabled to speed up the build start up
time.

Exception logging

See merge request !54

Add messages to exceptions and slightly refactor exception handling.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

Improve exception handling

See merge request !52

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Fix session import

See merge request !49

Readd an empty constructor for deserialization with Jackson. It was
accidentally removed a part of a code clean up in commit
685695a6.

Code cleanup

See merge request !48