gui, voting: Implement poll result caching and poll stale indicator #2806

Job	Run time
Lint	1m 17s
macOS 11 native [GOAL install] [GUI] [no depends]	22m 15s
ARM [GOAL install] [buster]	9m 17s
Win32	10m 49s
Win64	16m 16s
i386 Linux [GOAL install] [focal]	10m 34s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	25m 14s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	31m 37s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	15m 27s
	2h 22m 46s

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