fix: dev: Post [CVE-2024-12705] Performance Drop Fixes, Part 2

This merge request addresses several key performance bottlenecks in the DoH (DNS over HTTPS) implementation by introducing significant optimizations and improvements.

### Key Improvements

1. **Simplification and Optimisation of `http_do_bio()` Function**:
   - The code flow in the `http_do_bio()` function has been significantly simplified.
2. **Flushing HTTP Write Buffer on Outgoing DNS Messages**:
   - The buffer is flushed and a send operation is performed when there is an outgoing DNS message.
3. **Bumping Active Streams Processing Limit**:
   - The total number of active streams has been increased to 60% of the total streams limit.

These changes collectively enhance the performance and reliability of the DoH implementation, making it more efficient and robust for handling high-load scenarios, particularly noticeable in long runs (>= 1h) of `stress:long:rpz:doh+udp:linux:*` tests. It improves perf. for tests for BIND 9.18, but it likely will have a positive but less pronounced effect on newer versions as well.

In essence, the merge request fixes three bottlenecks stacked upon each other.

*It is a logical continuation of the merge requests !10109.* !10109, unfortunately, did not completely [address the performance drop in 9.18](https://gitlab.isc.org/isc-projects/bind9/-/pipelines/221545) for longer runs of the stress test. This merge request [addresses that](https://gitlab.isc.org/isc-projects/bind9/-/pipelines/223661).

**P.S.**

The origin of the fixes is, in fact, the branch in !10193. So this MR is a ... *forward port* of them.

Merge branch 'artem-doh-performance-drop-post-fix' into 'main'

See merge request isc-projects/bind9!10192