dbparser: Implement a template() option for the parser

Similar to the csv-parser(), this patch makes dbparser() accept a
template() option, that can be used to tell the parser what to parse
instead of the MESSAGE part (which it does by default).

This allows one to conveniently chain parsers together, assembling one
parser's output into an input template for another, without the need
for complicated rewrites:

,----
| parser p_step1 {
|  csv-parser(columns("timestamp", "host", "app", "instance", "session", "message"));
| };
|
| parser p_step2 {
|  db-parser(file("etc/patterndb.d/step2.xml")
|            template("${session}: ${message}"));
| };
|
| log {
|   source(...);
|   parser(p_step1);
|   parser(p_step2);
|   destination(...);
| };
`----

This patch makes this config work, so that the db-parser() will
operate on the given template, instead of $MESSAGE.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Preparations for 3.4.0rc2

NEWS & VERSION updated to 3.4.0rc2.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>