The Icinga DB daemon processes the data from the `IcingaApplication`
type only and Icinga DB Web also uses only those stats. However, before
this commit, Icinga DB published all kinds of useless stats to Redis
each second, like the number of (un)reachable hosts, services, and so
on, which is waste of CPU and some other resources. This commit reduces
the published data drastically to only those simple stats coming from
the `IcingaApplication` type.
This commit removes EmbeddedNamespaceValue and ConstEmbeddedNamespaceValue and
reduces NamespaceValue down to a simple struct without inheritance or member
functions. The code from these clases is inlined into the Namespace class. The
class hierarchy determining whether a value is const is moved to an attribute
of NamespaceValue.
This is done in preparation for changes to the locking in the Namespace class.
Currently, it relies on a recursive mutex. In the future, a shared mutex
(read/write lock) should be used instead, which cannot allow recursive locking
(without failing or risk deadlocking on lock upgrades). With this change, all
operations requiring a lock for one operation are within one function, no
recursive locking is not needed any more.
