Revert adding PerfdataWriterConnection

[jschmidt-icinga]

We still can't reproduce #10825 on our end or get any reliable info on what is causing the problem, so this reverts the merges of #10799 and #10668 on the 2.16 branch.

We are aware of #10848 and the locking issue discovered there, but for now concluded that rolling out a fix for that is too uncertain, without knowing the underlying issue of why the perfdata writer queues stop.

refs #10842 (this PR fixes it for support/2.16, issue remains open for master branch)
refs #10848 (this PR fixes it for support/2.16, issue remains open for master branch)
refs #10825 (this PR fixes it for support/2.16, issue remains open for master branch)

[julianbrost]

I've checked using git diff v2.15.3..revert-perfdatawriterconnection lib/perfdata/ that this PR reverts the perfdata writers to the state as of v2.15.3 with the following exception (at commit 103779d):

1. OTLPMetricsWriter is still there (which is unrelated to the writers causing problems).
2. Deprecation warning for ElasticsearchWriter is still there.
3. Minor changes to remove compiler warnings (making casts explicit, removing unused function parameters).
4. Changes of the file headers to use SPDX syntax.

[jschmidt-icinga]

with the following exception

The only real exception I made outside of solving those conflicts was also keeping the using namespace std::chrono_literals; in i2_base.hpp, since there were a few classes now requiring it. But that shouldn't really affect anything.

[jschmidt-icinga]

• Removed the disconnect_timeout attribute from the revert so user's that might have set these attribute will not get errors after the upgrade. The attributes will silently be ignored.