refactor(server): Cleanups in ua_subscription.h #1097

Job	Run time
Debug Build & Unit Tests (gcc)	2m 42s
Debug Build & Unit Tests (gcc, 32bit)	3m 26s
Debug Build & Unit Tests without Subscriptions (gcc)	1m 36s
Debug Build & Unit Tests with Diagnostics (gcc)	2m 38s
Debug Build & Unit Tests with multithreading (gcc)	1m 59s
Debug Build & Unit Tests with Alarms&Conditions (gcc)	11m 10s
Debug Build & Unit Tests (clang)	3m 50s
Debug Build & Unit Tests (tcc)	2m 15s
Encryption (MbedTLS) Build & Unit Tests (gcc)	2m 29s
PubSub Encryption (MbedTLS) Build & Unit Tests (gcc)	2m 49s
PubSub SKS Build & Unit Tests (gcc)	3m 18s
Encryption (OpenSSL) Build & Unit Tests (gcc)	2m 16s
Encryption (LibreSSL) Build & Unit Tests (gcc)	4m 47s
TPM Tool Build	2m 4s
Release Build	1m 2s
Amalgamation Build	43s
Amalgamation Build with Multithreading	45s
Valgrind Build & Unit Tests with MbedTLS (gcc)	38m 24s
Valgrind Build & Unit Tests with OpenSSL (gcc)	11m 51s
Valgrind Examples with MbedTLS (gcc)	58m 4s
Valgrind Examples with OpenSSL (gcc)	58m 8s
Clang Static Analyzer	3m 7s
Build All Companion Specifications	9m 2s
	3h 48m 25s

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