logmsg: add new interface to make easier to manipulate name value pairs.

There are a lot of occurrences in syslog-ng, where we have the name and
the value, but we have to get a handle and set or get the value via the handle

These functions make possible to omit to get the handle by name.

New functions:
  * log_msg_get_value_by_name
  * log_msg_set_value_by_name
logmsg: use the log_msg_set_value_by_name and log_msg_get_value_by_name

to make the code cleaner where it is possible.

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
Merge pull request #267 from juhaszviktor/f/reach_value_by_name

F/reach value by name