mongodb: Do not disable throttle()

Throttling was implemented in LogThrDestDriver, and as such, the MongoDB
driver gained support for it too. Therefore, we do not need to disable
it anymore.

This fixes #180.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
logthrdestdrv: Centralize the retries() functionality

Instead of implementing retries() for each driver, implement it once, in
LogThrDestDriver. This changes the API of worker->insert: it now
receives the message, and must return one of a number of resulting
states. The parent class will handle it from there. All derived drivers
were updated.

This fixes #201, and due to centralization and common behaviour, also
fixes #202 too.

As a side effect, this adds retries() functionality to amqp(), stomp()
and redis().

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
logthrdestdrv: Introduce threaded_dest_driver_option

This new rule in the grammar will hold all the options that are
centrally implemented by LogThrDestDriver. Currently, this is the
retries() option.

With the function moved to that class, the setter was moved there too,
and all copies were removed from the various drivers.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
cfg-parser: Move "retries" from afsql to cfg-parser

Now that "retries" is implemented by something in core too, move
associated things to cfg-parser.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>