[syslog-ng] MongoDB destination driver

[Gergely Nagy]

Another little update: I ported the mongodb destination driver from
using the mongodb C driver to the C++ driver, for a few reasons:

* The C driver had to be bundled with the source, which I dislike with
a passion.
* The C++ driver is much more mature, and a lot more tested aswell.

At the moment, there's a small bridge between the C and the C++ code,
neatly separated into two little files. Functionality remained the
same, stability hopefully improved, and there's less code to maintain
within the syslog-ng driver.

It's available on the algernon/dest/mongodb-cpp branch in my
repository - I haven't merged it onto the main algernon/dest/mongodb
branch just yet, there's a few little things I want to iron out first.
Not to mention that I'm not really sure about introducing a (partly)
C++ module to syslog-ng (even if it's optional, and not compiled by
default).