fix(pubsub): Fix mqtt reader group shutdown #68

Job	Run time
Debug Build & Unit Tests (gcc)	2m 58s
Debug Build & Unit Tests (gcc, 32bit)	4m 0s
Debug Build & Unit Tests without Subscriptions (gcc)	1m 19s
Debug Build & Unit Tests with Diagnostics (gcc)	2m 44s
Debug Build & Unit Tests with multithreading (gcc)	1m 52s
Debug Build & Unit Tests with Alarms&Conditions (gcc)	11m 29s
Debug Build & Unit Tests (clang)	3m 54s
Debug Build & Unit Tests (tcc)	2m 16s
Encryption (MbedTLS) Build & Unit Tests (gcc)	2m 22s
PubSub Encryption (MbedTLS) Build & Unit Tests (gcc)	3m 13s
PubSub SKS Build & Unit Tests (gcc)	3m 25s
Encryption (OpenSSL) Build & Unit Tests (gcc)	2m 3s
Encryption (LibreSSL) Build & Unit Tests (gcc)	4m 42s
TPM Tool Build	2m 25s
Release Build	1m 14s
Amalgamation Build	52s
Amalgamation Build with Multithreading	44s
Valgrind Build & Unit Tests with MbedTLS (gcc)	11m 27s
Valgrind Build & Unit Tests with OpenSSL (gcc)	11m 52s
Valgrind Examples with MbedTLS (gcc)	59m 36s
Valgrind Examples with OpenSSL (gcc)	59m 40s
Clang Static Analyzer	3m 10s
Build All Companion Specifications	9m 7s
	3h 26m 24s

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