This commit intruduces a small helper class that wraps any writer and
provides a flush operation that performs the corresponding action if the
writer is an AsyncJsonWriter and does nothing otherwise.
Replacing invalid UTF-8 characters beforehand by our selves doesn't make
any sense, the serializer can literally perform the same replacement ops
with the exact same Unicode replacement character (U+FFFD) on its own.
So, why not just use it directly? Instead of wasting memory on a temporary
`String` object to always UTF-8 validate every and each value, we just
use the serializer to directly to dump the replaced char (if any) into
the output writer. No memory waste, no fuss!
The Array, Dictionary, and Namespace types provide a Freeze() method that makes
them read-only. So far, there was the possibility to call some methods with
`overrideFrozen=true` which would then bypass the corresponding check and allow
modification of the data structures nonetheless.
With 24b57f0d3a222835178e88489eabd595755ed883, this possibility was already
removed from the Namespace type. However, for interface compatibility, it kept
the parameter and just ignores it, throwing an exception on any modification on
a frozen instance.
The only place using `overrideFrozen` was processing of the `-D`/`--define`
command line flag that allows setting additional variables in the DSL. At the
time it is evaluated, there are no user-created data structures yet that could
be frozen, so the only frozen objects that could be encountered are Namespaces
(Icinga doesn't freeze other types by itself) and for these, `overrideFrozen`
already has no effect.
Hence, there is no harm in removing `overrideFrozen` altogether. This
simplifies the code and also means that frozen objects are now indeed read-only
without exceptions, allowing further optimizations regarding locking in the
future.
Within the GNU/Linux distribution specific installation guides, the "Set
up Icinga DB" section was only excluded for openSUSE. However, since
there is an openSUSE installation guide within Icinga DB[^0], this is
not consistent. Thus, the if-guard was removed, resulting in this
section being available for each distribution. Windows is already
excluded through an if-guard above.
Some cases for Fedora were missing, which were also added.
[^0]: https://icinga.com/docs/icinga-db/latest/doc/02-Installation/openSUSE/
