Have Linux/MdnsImpl use System::Layer timer

[kpschoedel]

Problem

Linux/MdnsImpl currently uses ad-hoc coupling with the POSIX I/O event
loop, which intrudes on and constrains the latter's implementation.

Change overview

Replace the GetMdnsTimeout()/HandleMdnsTimeout() pair with a
System::Layer timer tracking the earliest mDNS timeout.

The last remnants of the Old Republic select() have been swept away.

Testing

Existing unit test TestMdns

[github-actions]

Size increase report for "esp32-example-build" from 3a50501

File	Section	File	VM
chip-temperature-measurement-app.elf	.flash.text	32	32
chip-shell.elf	.flash.text	-48	-48

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize
.flash.text,32,32
[Unmapped],0,-32

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
[Unmapped],0,48
.flash.text,-48,-48

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize