refactor(pubsub): Simplify WriterGroup state machine #988

Job	Run time
Debug Build & Unit Tests (gcc)	3m 27s
Debug Build & Unit Tests (gcc, 32bit)	3m 34s
Debug Build & Unit Tests without Subscriptions (gcc)	1m 56s
Debug Build & Unit Tests with Diagnostics (gcc)	3m 30s
Debug Build & Unit Tests with multithreading (gcc)	2m 33s
Debug Build & Unit Tests with Alarms&Conditions (gcc)	14m 1s
Debug Build & Unit Tests (clang)	5m 0s
Debug Build & Unit Tests (tcc)	3m 18s
Encryption (MbedTLS) Build & Unit Tests (gcc)	2m 51s
PubSub Encryption (MbedTLS) Build & Unit Tests (gcc)	3m 42s
PubSub SKS Build & Unit Tests (gcc)	5m 13s
Encryption (OpenSSL) Build & Unit Tests (gcc)	2m 40s
Encryption (LibreSSL) Build & Unit Tests (gcc)	6m 20s
TPM Tool Build	3m 49s
Release Build	1m 28s
Amalgamation Build	1m 0s
Amalgamation Build with Multithreading	59s
Valgrind Build & Unit Tests with MbedTLS (gcc)	16m 14s
Valgrind Build & Unit Tests with OpenSSL (gcc)	16m 40s
Valgrind Examples with MbedTLS (gcc)	1m 30s
Valgrind Examples with OpenSSL (gcc)	1m 27s
Clang Static Analyzer	5m 9s
Build All Companion Specifications	14m 54s
	2h 1m 15s

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