refactor(pubsub): Simplify WriterGroup state machine #987

Job	Run time
Debug Build & Unit Tests (gcc)	51s
Debug Build & Unit Tests (gcc, 32bit)	1m 35s
Debug Build & Unit Tests without Subscriptions (gcc)	1m 57s
Debug Build & Unit Tests with Diagnostics (gcc)	1m 2s
Debug Build & Unit Tests with multithreading (gcc)	2m 30s
Debug Build & Unit Tests with Alarms&Conditions (gcc)	12m 55s
Debug Build & Unit Tests (clang)	59s
Debug Build & Unit Tests (tcc)	50s
Encryption (MbedTLS) Build & Unit Tests (gcc)	2m 42s
PubSub Encryption (MbedTLS) Build & Unit Tests (gcc)	51s
PubSub SKS Build & Unit Tests (gcc)	1m 24s
Encryption (OpenSSL) Build & Unit Tests (gcc)	2m 40s
Encryption (LibreSSL) Build & Unit Tests (gcc)	6m 13s
TPM Tool Build	3m 5s
Release Build	1m 29s
Amalgamation Build	53s
Amalgamation Build with Multithreading	1m 10s
Valgrind Build & Unit Tests with MbedTLS (gcc)	1m 2s
Valgrind Build & Unit Tests with OpenSSL (gcc)	1m 12s
Valgrind Examples with MbedTLS (gcc)	1m 28s
Valgrind Examples with OpenSSL (gcc)	1m 17s
Clang Static Analyzer	2m 8s
Build All Companion Specifications	14m 38s
	1h 4m 51s

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