stomp: New STOMP destination driver

Based on initial work by Attila Nagy <bra@fsn.hu>, this destination adds
support for sending events to any STOMP-capable message queue. Similar
to AMQP and MongoDB, the driver uses value-pairs to transfer messages
across. An optional body can still be set, however.

destination d_stomp {
 stomp(
  host("localhost")
  port(61613)
  destination("/topic/syslog")
  routing-key("")      # optional, empty by default
  body("")             # optional, empty by default
  persistent(yes)
  ack(no)
  username("user")     # optional, empty by default
  password("password") # optional, empty by default
  value-pairs(scope(selected-macros, nv-pairs, sdata))
 );
};

The driver requires no new dependencies, an internal STOMP client
implementation is used.

Signed-off-by: Attila Nagy <bra@fsn.hu>
Signed-off-by: Viktor Tusa <tusa@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>